Ensure ordering of overlay callbacks

Before this CL, the DreamOverlayService in SysUI used an executor to
run code in callbacks from the base class. However, this was not
synchronized with the endDream call coming from OverlayClient in the
base class. This meant that an endDream call could come in before or
during the onStartDream implementation in SysUI, causing calls like
shouldShowComplications and getDreamComponent to throw an exception
since the overlay client was already disconnected.

This change allows providing an executor to the base
DreamOverlayService class that it will use to run callbacks coming
from the OverlayClient over binder. This makes it so that endDream
will not run before the onStartDream callback's contents run.

Bug: 271374409
Test: atest DreamOverlayServiceTest
Change-Id: I35a0badfc401d5224943e71fcc241b8a32698bea