Android: add support for headless js tasks

Summary: Provide a base `HeadlessJsTaskService` class that can be extended to run JS in headless mode in response to some event. Added `HeadlessJsTaskEventListener` for modules that are interested in background lifecycle events, and `HeadlessJsTaskContext` that basically extends `ReactContext` without touching it. The react instance is shared with the rest of the app (e.g. activities) through the `ReactNativeHost`.

Reviewed By: astreet

Differential Revision: D3225753

fbshipit-source-id: 2c5e7679636f31e0e7842d8a67aeb95baf47c563

ReactAndroid/src/main/java/com/facebook/react/jstasks/BUCK

@@ -0,0 +1,23 @@
+include_defs('//ReactAndroid/DEFS')
+
+DEPS = [
+  react_native_target('java/com/facebook/react/bridge:bridge'),
+  react_native_target('java/com/facebook/react/common:common'),
+  react_native_target('java/com/facebook/react/uimanager:uimanager'),
+  react_native_dep('libraries/fbcore/src/main/java/com/facebook/common/logging:logging'),
+  react_native_dep('third-party/java/infer-annotations:infer-annotations'),
+  react_native_dep('third-party/java/jsr-305:jsr-305'),
+]
+
+android_library(
+  name = 'jstasks',
+  srcs = glob(['*.java']),
+  deps = DEPS,
+  visibility = [
+    'PUBLIC',
+  ],
+)
+
+project_config(
+  src_target = ':jstasks',
+)