Allow developers to load JavaScript bundle from any source.

Summary: This patch adds two pieces of functionality:

- Exposes `JSBundleLoader` to allow a developer to load JavaScript bundles as they choose.
- Adds `ReactBridge.loadScripFromFile` method which loads a JavaScript bundle from an arbitrary file path.

Example usage:

```
JSBundleLoader jsBundleLoader = new JSBundleLoader() {
    Override
    public void loadScript(ReactBridge reactBridge) {
        reactBridge.loadScriptFromFile("/sdcard/Download/index.android.bundle");
    }
};

mReactInstanceManager = ReactInstanceManager.builder()
        .setApplication(getApplication())
        .setJSBundleLoader(jsBundleLoader)
        .setJSMainModuleName("") /* necessary due to TODO(6803830) */
        .addPackage(new MainReactPackage())
        .setInitialLifecycleState(LifecycleState.RESUMED)
        .build();
```

cc ide
Closes https://github.com/facebook/react-native/pull/3189

Reviewed By: svcscm

Differential Revision: D2535819

Pulled By: mkonicek

fb-gh-sync-id: f319299dbe29bab3b7e91f94249c14b270d9fec3

ReactAndroid/src/main/java/com/facebook/react/bridge/JSBundleLoader.java

@@ -9,7 +9,7 @@
 
 package com.facebook.react.bridge;
 
-import android.content.res.AssetManager;
+import android.content.Context;
 
 /**
  * A class that stores JS bundle information and allows {@link CatalystInstance} to load a correct
@@ -22,18 +22,22 @@ public abstract class JSBundleLoader {
    * should be used. JS bundle will be read from assets directory in native code to save on passing
    * large strings from java to native memory.
    */
-  public static JSBundleLoader createAssetLoader(
-      final AssetManager assetManager,
-      final String assetFileName) {
+  public static JSBundleLoader createFileLoader(
+      final Context context,
+      final String fileName) {
     return new JSBundleLoader() {
       @Override
       public void loadScript(ReactBridge bridge) {
-        bridge.loadScriptFromAssets(assetManager, assetFileName);
+        if (fileName.startsWith("assets://")) {
+          bridge.loadScriptFromAssets(context.getAssets(), fileName.replaceFirst("assets://", ""));
+        } else {
+          bridge.loadScriptFromFile(fileName, fileName);
+        }
       }
 
       @Override
       public String getSourceUrl() {
-        return "file:///android_asset/" + assetFileName;
+        return fileName;
       }
     };
   }
@@ -51,7 +55,7 @@ public abstract class JSBundleLoader {
     return new JSBundleLoader() {
       @Override
       public void loadScript(ReactBridge bridge) {
-        bridge.loadScriptFromNetworkCached(sourceURL, cachedFileLocation);
+        bridge.loadScriptFromFile(cachedFileLocation, sourceURL);
       }
 
       @Override
@@ -70,7 +74,7 @@ public abstract class JSBundleLoader {
     return new JSBundleLoader() {
       @Override
       public void loadScript(ReactBridge bridge) {
-        bridge.loadScriptFromNetworkCached(sourceURL, null);
+        bridge.loadScriptFromFile(null, sourceURL);
       }
 
       @Override