Reduce flakiness of TaskStackChangedListenerTest

It is more stable to use count down latch to verify, instead of waiting
for main thread idle.

Also simplify testTaskStackChanged_afterFinish that it doesn't need
to start 2 activities which may need to handle more races. Just verify
the result after finishing the current one.

Bug: 130388819
Bug: 154640614
Test: TaskStackChangedListenerTest#testTaskStackChanged_afterFinish
   TaskStackChangedListenerTest#testTaskStackChanged_resumeWhilePausing
Change-Id: I7e88e7f1f470b04cad8a348932bb784bf8f3e5f0