Wait for the Activity to be destroyed before verifying

The test expect that the activities above the target activity should
be destroyed, so we should wait for these activities to be destroyed
before verifying.

Bug: 172595632
Test: atest ActivityStarterTests#testActivityWithClearTaskOnLaunch
Change-Id: Ibd1fc0ce0f83f4d8827ca576c3cc9967867157e6