Add onStart hook to FlutterFragmentActivity (#6719)

Seems like this file was missed when onStart hooks were added recently in this pull request: https://github.com/flutter/engine/pull/5177

shell/platform/android/io/flutter/app/FlutterFragmentActivity.java

@@ -102,6 +102,12 @@ public class FlutterFragmentActivity
         }
     }
         
+    @Override
+    protected void onStart() {
+        super.onStart();
+        eventDelegate.onStart();
+    }
+
     @Override
     protected void onStop() {
         eventDelegate.onStop();