cbetter javadoc and comments

Author: leotianlizhan

firebase-perf/src/main/java/com/google/firebase/perf/application/AppStateMonitor.java

@@ -166,7 +166,9 @@ public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
 
   @Override
   public void onActivityDestroyed(Activity activity) {
+    // Dereference FrameMetricsRecorder from the map because it holds an Activity reference
     activityToRecorderMap.remove(activity);
+    // Dereference FragmentStateMonitor because it holds a FrameMetricsRecorder reference
     if (activityToFragmentStateMonitorMap.containsKey(activity)) {
       FragmentActivity fragmentActivity = (FragmentActivity) activity;
       fragmentActivity