Fix VoicemailPlaybackTest flakiness.

Change-Id: I9568ab3236ab02125c3d7da5f8a477df34bd7499
diff --git a/tests/src/com/android/dialer/voicemail/VoicemailPlaybackTest.java b/tests/src/com/android/dialer/voicemail/VoicemailPlaybackTest.java
index dd86b0d..96ef62a 100644
--- a/tests/src/com/android/dialer/voicemail/VoicemailPlaybackTest.java
+++ b/tests/src/com/android/dialer/voicemail/VoicemailPlaybackTest.java
@@ -109,6 +109,7 @@
         // There is a background check that is testing to see if we have the content available.
         // Once that task completes, we shouldn't be showing the fetching message.
         mFakeAsyncTaskExecutor.runTask(CHECK_FOR_CONTENT);
+        getInstrumentation().waitForIdleSync();
 
         assertHasOneTextViewContaining("Buffering");
         assertHasZeroTextViewsContaining("Loading voicemail");
@@ -119,6 +120,7 @@
         setPlaybackViewForPresenter();
 
         mFakeAsyncTaskExecutor.runTask(CHECK_FOR_CONTENT);
+        getInstrumentation().waitForIdleSync();
 
         // The media player will have thrown an IOException since the file doesn't exist.
         // This should have put a failed to play message on screen, buffering is gone.