Allow ReactActivityDelegate subclass to overrride mainComponentName (#19814)

Summary:
Currently, the final field mMainComponentName is used. This field is
initialized in the constructor of ReactActivityDelegate, and the
ReactActivityDelegate itself is initialized in the constructor of
ReactActivity. This means that the only way you can pass a main
component name to ReactActivityDelegate, is when your ReactActivity
subclass is constructed. At this point in the lifecycle of an
activity, the getIntent() value that the activity was initialized by
returns null, making it impossible to set the mainComponentName
dynamically based on data passed to the activity via an intent.

The mMainComponentName final field is also only used in onCreate of
the delegate, so it's not actually needed by the ReactActivityDelegate
at construction time. So the above limitation is not fundamental, it's
just a side effect of the API design.

By allowing subclasses of ReactActivityDelegate to implement a
getMainComponentName method, the subclass then has full control of how
to initialize the value. So an implementation of getMainComponentName
could be:

    public String getMainComponentName() {
        return getIntent().getStringExtra("reactMainComponentName");
    }

This commit doesn't remove anything and only adds a new method, so it
should be fully backwards compatible.

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Pull Request resolved: https://github.com/facebook/react-native/pull/19814

Differential Revision: D14206644

Pulled By: cpojer

fbshipit-source-id: 8532560f944362fe0cce263a7f9a503df8ae539f

ReactAndroid/src/main/java/com/facebook/react/ReactActivityDelegate.java

@@ -70,9 +70,14 @@ public class ReactActivityDelegate {
     return getReactNativeHost().getReactInstanceManager();
   }
 
+  public String getMainComponentName() {
+    return mMainComponentName;
+  }
+
   protected void onCreate(Bundle savedInstanceState) {
-    if (mMainComponentName != null) {
+    String mainComponentName = getMainComponentName();
-      loadApp(mMainComponentName);
+    if (mainComponentName != null) {
+      loadApp(mainComponentName);
     }
     mDoubleTapReloadRecognizer = new DoubleTapReloadRecognizer();
   }