Add unit test

Author: tejasd

firebase-perf/src/main/java/com/google/firebase/perf/FirebasePerfEarly.java

@@ -48,7 +48,6 @@ public FirebasePerfEarly(
     if (startupTime != null) {
       AppStartTrace appStartTrace = AppStartTrace.getInstance();
       appStartTrace.registerActivityLifecycleCallbacks(context);
-      // TODO(b/339891952): Investigate why this runs *before* the activity lifecycle callbacks.
       uiExecutor.execute(new AppStartTrace.StartFromBackgroundRunnable(appStartTrace));
     }
 

firebase-perf/src/main/java/com/google/firebase/perf/metrics/AppStartTrace.java

@@ -638,4 +638,9 @@ Timer getOnResumeTime() {
   void setIsStartFromBackground() {
     isStartedFromBackground = true;
   }
+
+  @VisibleForTesting
+  void setMainThreadRunnableTime(Timer timer) {
+    mainThreadRunnableTime = timer;
+  }
 }

firebase-perf/src/test/java/com/google/firebase/perf/metrics/AppStartTraceTest.java

@@ -18,6 +18,7 @@
 import static org.mockito.ArgumentMatchers.isA;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -258,6 +259,34 @@ public void testStartFromBackground() {
             ArgumentMatchers.nullable(ApplicationProcessState.class));
   }
 
+  @Test
+  public void testStartFromBackground_invertedOrder() {
+    FakeScheduledExecutorService fakeExecutorService = new FakeScheduledExecutorService();
+    Timer fakeTimer = spy(new Timer(currentTime));
+    AppStartTrace trace =
+        new AppStartTrace(transportManager, clock, configResolver, fakeExecutorService);
+    trace.registerActivityLifecycleCallbacks(appContext);
+    trace.setIsStartFromBackground();
+    trace.setMainThreadRunnableTime(fakeTimer);
+
+    when(fakeTimer.getDurationMicros()).thenReturn(99L);
+    trace.onActivityCreated(activity1, bundle);
+    Assert.assertNotNull(trace.getOnCreateTime());
+    ++currentTime;
+    trace.onActivityStarted(activity1);
+    Assert.assertNotNull(trace.getOnStartTime());
+    ++currentTime;
+    trace.onActivityResumed(activity1);
+    Assert.assertNotNull(trace.getOnResumeTime());
+    // There should be a trace sent since the delay between the main thread and onActivityCreated
+    // is limited.
+    fakeExecutorService.runAll();
+    verify(transportManager, times(1))
+        .log(
+            traceArgumentCaptor.capture(),
+            ArgumentMatchers.nullable(ApplicationProcessState.class));
+  }
+
   @Test
   @Config(sdk = 26)
   public void timeToInitialDisplay_isLogged() {