Changed UserVisibilityMediatorTestCase to use the main handler.

This test was creating a new handler thread as the goal is to make
sure the listener is called in the proper handler, but using the
test's main handler is enough (not to mention that sometimes the
new handler thread fails to start, which seems to be a test setup
issue).

Also decreased waiting time for no-events callbacks - given that
these are unit tests, it's ok to be aggressive and keep them small.

Bug: 259906148
Test: time atest --rerun-until-failure 100 FrameworksMockingServicesTests:com.android.server.pm.UserVisibilityMediatorSUSDTest FrameworksMockingServicesTests:com.android.server.pm.UserVisibilityMediatorMUMDTest

Change-Id: I0a144d60d12c623da7036501071a767e5a152af9