ReactActivity extends FragmentActivity (#22662)

Summary:
In facebook/react-native#20602, I tried to make ReactActivity to extend AppCompatActivity per Google recommendation. But import failed, now ReactActivity extends FragmentActivity which is a parent class of AppCompatActivity and step forward to extend AppCompatActivity.
Pull Request resolved: facebook/react-native#22662

Reviewed By: mdvacca

Differential Revision: D13505140

Pulled By: hramos

fbshipit-source-id: d4edc8dc5c606c45811c1deddf5727a47ad484d8

android/app/src/main/java/com/facebook/react/uiapp/RNTesterActivity.java

@@ -20,9 +20,9 @@ public class RNTesterActivity extends ReactActivity {
   public static class RNTesterActivityDelegate extends ReactActivityDelegate {
     private static final String PARAM_ROUTE = "route";
     private Bundle mInitialProps = null;
-    private final @Nullable Activity mActivity;
+    private final @Nullable ReactActivity mActivity;
 
-    public RNTesterActivityDelegate(Activity activity, String mainComponentName) {
+    public RNTesterActivityDelegate(ReactActivity activity, String mainComponentName) {
       super(activity, mainComponentName);
       this.mActivity = activity;
     }