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/devsupport/DevSupportManager.java

@@ -367,10 +367,18 @@ public class DevSupportManager implements NativeModuleCallExceptionHandler {
   }
 
   public String getSourceMapUrl() {
+    if (mJSAppBundleName == null) {
+      return "";
+    }
+
     return mDevServerHelper.getSourceMapUrl(Assertions.assertNotNull(mJSAppBundleName));
   }
 
   public String getSourceUrl() {
+    if (mJSAppBundleName == null) {
+      return "";
+    }
+
     return mDevServerHelper.getSourceUrl(Assertions.assertNotNull(mJSAppBundleName));
   }