Return to dream on power button press when occluded by glanceable hub.

This change also tracks focus for glanceable hub so the dream knows when
it is occluded.

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

Change-Id: I906b4e9d391173d80c114872a987eec578573c36