Merge pull request #2801 from corbt/back_button_android

handle back button press in JS

local-cli/generator-android/templates/package/MainActivity.java

@@ -44,6 +44,15 @@ public class MainActivity extends Activity implements DefaultHardwareBackBtnHand
         return super.onKeyUp(keyCode, event);
     }
 
+    @Override
+    public void onBackPressed() {
+      if (mReactInstanceManager != null) {
+        mReactInstanceManager.onBackPressed();
+      } else {
+        super.onBackPressed();
+      }
+    }
+
     @Override
     public void invokeDefaultOnBackPressed() {
       super.onBackPressed();