Manually track dreams being obscured.

This change introduces a new IDreamManager method to allow SysUI to
report when a dream is obscured by something (notification shade,
bouncer, etc.). This allows handling a power button tap to bring the
dream back to the front (rather than turning the display off).

This change also removes a previous attempt to use focus to achieve the
same effect (focus ended up not quite working in all cases).

Bug: 329125239, 337302237
Test: atest DreamOverlayServiceTest
Test: atest DreamOverlayContainerViewControllerTest
Test: atest DreamServiceTests
Flag: ACONFIG android.service.dreams.dream_handles_being_obscured STAGING
Change-Id: I35d9a0dff492443fa2e659b7ed442b8e4e3c641b