Tie dream lifecycle to dream overlay

Currently, when the overlay terminates, we attempt to keep the dream
active and restart the overlay connection. However, WindowManager seems
to now remove the dream window when the overlay process dies. Therefore
there is no longer a need to try to keep the dream running, and we can
tie the dream lifecycle to the overlay.

With this change, the dream is finished when the overlay is
disconnected. This ensures that we reset the dream state if SystemUI
crashes.

Fixes: 343506030
Flag: EXEMPT bugfix
Test: atest DreamServiceTest
Test: atest DreamOverlayConnectionHandlerTest
Change-Id: Ie8c3caf80b01cb1bf9d6127fe56d94d73ed59703