Wait a while before canceling the notification

If the notification in the enqueued list that hasn't had
a chance to post yet, the cancel operation will not
really do until it posted.
Wait for the notification to be posted before canceling it.

Bug: 160446933
Test: run cts -m CtsAppTestCases -t
android.app.cts.NotificationManagerTest#testCancel

Change-Id: I7a80410eac1a8b5497d7aa71e78a2ce90a7a9455
1 file changed