TestLooperManager in DisplayManagerServiceTest

Make sure that both the display looper and the power looper are flushed. Don't use runWithScissors because it will create a deadlock if used with a looper that's managed by a TestLooperManager.

Bug: 416687474
Test: DisplayManagerServiceTest
Flag: EXEMPT bugfix
Change-Id: Ie5c2fee7af2c11f25a934859d2dbb490d38391b4