Cleanup pending Handler messages after test finished

Hopefully this mitigates the presubmit failure frequency.

This partially reverts commit 809cbc53 and
thus re-open issue 121229219.

Bug: 121229219
Test: Pass all presubmit tests for Window manager.
  $ atest --test-mapping frameworks/base/services/core/java/com/android/server/wm

Change-Id: I7033fc0a37e2900c5dfd991fc2ab4e840d0e0a38