Track dream focus in order to know when dreams are occluded.

One of the changes needed to return to a dream on power button press if
the dream is occluded by something like the notification shade or the
bouncer.

Bug: 331798001
Test: atest DreamServiceTests
Flag: ACONFIG android.service.dreams.dream_tracks_focus STAGING

Change-Id: Iebb5470b7bdda869d46cdb7ffb6bedb580810ae5