Disallow unchecked message removal on shared handler

The dispose() of each shared handler threads in
SystemServicesTestRule#tearDown should be enough for test cleanup.

So remove the usages of removeCallbacksAndMessages to align the
policy of shared handler.

Bug: 235594693
Test: atest WmTests
Change-Id: I4662f06d1d45c62bba08f286643df17c39eec43d