Fix DreamService -> DreamOverlayService synchronization issues.

This change moves the DreamOverlayService connection management logic
out of DreamService so that it can be separately tested. It also updates
the connection logic to use a handler for synchronization of messages,
and correctly cancels pending messages when needed.

This avoids consumers from running after the service has been
unbound.

Fixes: 267335683
Test: atest DreamOverlayConnectionHandlerTest
Test: flashed device and verified no crash when enabling/disabling
lowlight, and pressing the power button

Change-Id: I93855885ad80410693b8dcb4c2e696e0cabc5737