move screen trace start to onResum

Author: leotianlizhan

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

@@ -146,20 +146,7 @@ public void onActivityCreated(Activity activity, Bundle savedInstanceState) {}
   public void onActivityDestroyed(Activity activity) {}
 
   @Override
-  public synchronized void onActivityStarted(Activity activity) {
-    if (isScreenTraceSupported(activity) && configResolver.isPerformanceMonitoringEnabled()) {
-      // Starts recording frame metrics for this activity.
-      /**
-       * TODO: Only add activities that are hardware acceleration enabled so that calling {@link
-       * FrameMetricsAggregator#remove(Activity)} will not throw exceptions.
-       */
-      frameMetricsAggregator.add(activity);
-      // Start the Trace
-      Trace screenTrace = new Trace(getScreenTraceName(activity), transportManager, clock, this);
-      screenTrace.start();
-      activityToScreenTraceMap.put(activity, screenTrace);
-    }
-  }
+  public synchronized void onActivityStarted(Activity activity) {}
 
   @Override
   public synchronized void onActivityStopped(Activity activity) {
@@ -181,7 +168,7 @@ public synchronized void onActivityResumed(Activity activity) {
     // cases:
     // 1. At app startup, first activity comes to foreground.
     // 2. app switch from background to foreground.
-    // 3. app already in foreground, current activity is replaced by another activity.
+    // 3. app already in foreground, current activity is replaced by another activity, or the current activity was paused then resumed without onStop (ex: a dialog pauses the activity, then closing it resumes)
     if (activityToResumedMap.isEmpty()) {
       // The first resumed activity means app comes to foreground.
       resumeTime = clock.getTime();
@@ -198,9 +185,22 @@ public synchronized void onActivityResumed(Activity activity) {
         updateAppState(ApplicationProcessState.FOREGROUND);
       }
     } else {
-      // case 3: app already in foreground, current activity is replaced by another activity.
+      // case 3: app already in foreground, current activity is replaced by another activity, or the current activity was paused then resumed without onStop (ex: a dialog pauses the activity, then closing it resumes)
       activityToResumedMap.put(activity, true);
     }
+
+    if (isScreenTraceSupported(activity) && configResolver.isPerformanceMonitoringEnabled()) {
+      // Starts recording frame metrics for this activity.
+      /**
+       * TODO: Only add activities that are hardware acceleration enabled so that calling {@link
+       * FrameMetricsAggregator#remove(Activity)} will not throw exceptions.
+       */
+      frameMetricsAggregator.add(activity);
+      // Start the Trace
+      Trace screenTrace = new Trace(getScreenTraceName(activity), transportManager, clock, this);
+      screenTrace.start();
+      activityToScreenTraceMap.put(activity, screenTrace);
+    }
   }
 
   /** Returns if this is the cold start of the app. */