Get RNTester to compile and run

Summary:
This diff includes a few changes:
1. Move the headers inside `jsiexecutor` into `jsiexecutor/jsireact`. As far as I'm aware, the Android ndk build system isn't flexible enough to support header namespaces, so we can't just expose the headers inside the `jsiexecutor` directory under the `jsireact` namespace. Therefore, I moved the headers to `jsiexecutor/jsireact`, and added `jsiexecutor` to the header search path.  This was the easiest way to simulate `jsireact` namespace.
2. Setup the Android.mk files to get RNTester compiling and running.
3. Introduce a `jscexecutor` module to make `JSCExecutor.java` execute without throwing.

**Note:** Moving the header files inside `jsiexecutor` probably breaks the iOS builds and internal builds. I'll fix those in subsequent diffs on this stack.

Reviewed By: shergin

Differential Revision: D9995429

fbshipit-source-id: 418a4ee91f585842c5e317af2f300227a51e9ba8

ReactAndroid/build.gradle

@@ -129,7 +129,7 @@ task prepareGlog(dependsOn: dependenciesPath ? [] : [downloadGlog], type: Copy)
 task downloadJSCHeaders(type: Download) {
     // in sync with webkit SVN revision 174650
     def jscAPIBaseURL = 'https://raw.githubusercontent.com/WebKit/webkit/38b15a3ba3c1b0798f2036f7cea36ffdc096202e/Source/JavaScriptCore/API/'
-    def jscHeaderFiles = ['JavaScript.h', 'JSBase.h', 'JSContextRef.h', 'JSObjectRef.h', 'JSRetainPtr.h', 'JSStringRef.h', 'JSValueRef.h', 'WebKitAvailability.h']
+    def jscHeaderFiles = ['JavaScript.h', 'JSBase.h', 'JSContextRef.h', 'JSObjectRef.h', 'JSStringRef.h', 'JSValueRef.h', 'WebKitAvailability.h']
     def output = new File(downloadsDir, 'jsc')
     output.mkdirs()
     src(jscHeaderFiles.collect { headerName -> "$jscAPIBaseURL$headerName" })
@@ -221,6 +221,7 @@ task buildReactNdkLib(dependsOn: [prepareJSC, prepareBoost, prepareDoubleConvers
             "NDK_LIBS_OUT=$buildDir/react-ndk/all",
             "THIRD_PARTY_NDK_DIR=$buildDir/third-party-ndk",
             "REACT_COMMON_DIR=$projectDir/../ReactCommon",
+            "REACT_SRC_DIR=$projectDir/src/main/java/com/facebook/react",
             '-C', file('src/main/jni/react/jni').absolutePath,
             '--jobs', project.hasProperty("jobs") ? project.property("jobs") : Runtime.runtime.availableProcessors()
 }

ReactAndroid/src/main/java/com/facebook/react/jscexecutor/Android.mk

@@ -0,0 +1,16 @@
+LOCAL_PATH := $(call my-dir)
+
+include $(CLEAR_VARS)
+
+LOCAL_MODULE := jscexecutor
+
+LOCAL_SRC_FILES := $(wildcard $(LOCAL_PATH)/*.cpp)
+
+LOCAL_C_INCLUDES := $(LOCAL_PATH)
+
+LOCAL_CFLAGS += -fvisibility=hidden -fexceptions -frtti
+
+LOCAL_STATIC_LIBRARIES := libjsi libjsireact
+LOCAL_SHARED_LIBRARIES := libfolly_json libfb libreactnativejni
+
+include $(BUILD_SHARED_LIBRARY)

ReactAndroid/src/main/jni/Application.mk

@@ -5,7 +5,19 @@ APP_PLATFORM := android-16
 
 APP_MK_DIR := $(dir $(lastword $(MAKEFILE_LIST)))
 
-NDK_MODULE_PATH := $(APP_MK_DIR)$(HOST_DIRSEP)$(THIRD_PARTY_NDK_DIR)$(HOST_DIRSEP)$(REACT_COMMON_DIR)$(HOST_DIRSEP)$(APP_MK_DIR)first-party
+# What is NDK_MODULE_PATH?
+#   This is comparable to the PATH environment variable in Linux. The purpose
+#   of NDK_MODULE_PATH is to provide a list of directories that contain modules
+#   we want ndk-build to compile.
+#
+# What is HOST_DIRSEP?
+#   In PATH, the directories are separated by a ':'.
+#   In NDK_MODULE_PATH, the directories are separated by $(HOST_DIRSEP).
+#
+# Where are APP_MK_DIR, THIRD_PARTY_NDK_DIR, etc. defined?
+#   The directories inside NDK_MODULE_PATH (ex: APP_MK_DIR, THIRD_PARTY_NDK_DIR,
+#   etc.) are defined inside build.gradle.
+NDK_MODULE_PATH := $(APP_MK_DIR)$(HOST_DIRSEP)$(THIRD_PARTY_NDK_DIR)$(HOST_DIRSEP)$(REACT_COMMON_DIR)$(HOST_DIRSEP)$(APP_MK_DIR)first-party$(HOST_DIRSEP)$(REACT_SRC_DIR)
 
 APP_STL := gnustl_shared
 

ReactAndroid/src/main/jni/react/jni/Android.mk

@@ -2,47 +2,61 @@ LOCAL_PATH := $(call my-dir)
 
 include $(CLEAR_VARS)
 
-LOCAL_MODULE := reactnativejni
-
-LOCAL_SRC_FILES := \
-  AndroidJSCFactory.cpp \
-  CatalystInstanceImpl.cpp \
-  CxxModuleWrapper.cpp \
-  JavaModuleWrapper.cpp \
-  JReactMarker.cpp \
-  JMessageQueueThread.cpp \
-  JSCPerfLogging.cpp \
-  JSLoader.cpp \
-  JSLogging.cpp \
-  JniJSModulesUnbundle.cpp \
-  MethodInvoker.cpp \
-  ModuleRegistryBuilder.cpp \
-  NativeArray.cpp \
-  NativeCommon.cpp \
-  NativeDeltaClient.cpp \
-  NativeMap.cpp \
-  OnLoad.cpp \
-  ProxyExecutor.cpp \
-  ReadableNativeArray.cpp \
-  ReadableNativeMap.cpp \
-  WritableNativeArray.cpp \
-  WritableNativeMap.cpp \
-
+# Include . in the header search path for all source files in this module.
 LOCAL_C_INCLUDES := $(LOCAL_PATH)
+
+# Include ./../../ in the header search path for modules that depend on
+# reactnativejni. This will allow external modules to require this module's
+# headers using #include <react/jni/<header>.h>, assuming:
+#   .     == jni
+#   ./../ == react
 LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/../..
 
-LOCAL_CFLAGS += -fvisibility=hidden -fexceptions -frtti
+LOCAL_CFLAGS += -fexceptions -frtti
 
 LOCAL_LDLIBS += -landroid
-LOCAL_SHARED_LIBRARIES := libfolly_json libfb libjsc libglog_init libyoga libprivatedata
+
+# The dynamic libraries (.so files) that this module depends on.
+LOCAL_SHARED_LIBRARIES := libfolly_json libfb libjsc libglog_init libyoga
+
+# The static libraries (.a files) that this module depends on.
 LOCAL_STATIC_LIBRARIES := libreactnative
 
+# Name of this module.
+#
+# Other modules can depend on this one by adding libreactnativejni to their
+# LOCAL_SHARED_LIBRARIES variable.
+LOCAL_MODULE := reactnativejni
+
+# Compile all local c++ files.
+LOCAL_SRC_FILES := $(wildcard *.cpp)
+
+# Build the files in this directory as a shared library
 include $(BUILD_SHARED_LIBRARY)
 
-$(call import-module,cxxreact)
-$(call import-module,privatedata)
-$(call import-module,fb)
-$(call import-module,fbgloginit)
+# Compile the c++ dependencies required for ReactAndroid
+#
+# How does the import-module function work?
+#   For each $(call import-module,<module-dir>), you search the directories in
+#   NDK_MODULE_PATH. (This variable is defined in Application.mk). If you find a
+#   <module-dir>/Android.mk you in a directory <dir>, you run:
+#   include <dir>/<module-dir>/Android.mk
+#
+# What does it mean to include an Android.mk file?
+#   Whenever you encounter an include <dir>/<module-dir>/Android.mk, you
+#   tell andorid-ndk to compile the module in <dir>/<module-dir> according
+#   to the specification inside <dir>/<module-dir>/Android.mk.
 $(call import-module,folly)
+$(call import-module,fb)
 $(call import-module,jsc)
+$(call import-module,fbgloginit)
 $(call import-module,yogajni)
+$(call import-module,cxxreact)
+$(call import-module,jsi)
+$(call import-module,jsiexecutor)
+
+# TODO(ramanpreet):
+#   Why doesn't this import-module call generate a jscexecutor.so file?
+# $(call import-module,jscexecutor)
+
+include $(REACT_SRC_DIR)/jscexecutor/Android.mk