Don't rely on WaitForIdle when possible

In many tests, we can wait for a known signal to be received instead of
waiting for the system to be idle. This CL update many of the tests to
wait for that signal.

Test: atest DisplayServiceTests:com.android.server.display.DisplayManagerServiceTest
Fix: 298160167
Change-Id: Ic405b70bf55c40d3087efc69d75d6c6478bc695a