Do not stop dream service if dream activity is relaunching

Currently DreamActivity doesn't handle any configuration changes,
and does not set fixed orientation. So the decor view will be
detached if the activity is relaunching for configuration changes.

Though it also works by making DreamActivity support handling the
changes, all implementations of DreamService may need to add the
corresponding handling. So this fix only keeps the service alive
for compatibility.

Fixes: 154474768
Test: atest DreamManagerServiceTests#testDreamNotFinishAfterRotation
Test: Enter Settings > Display > Screen saver, press "Start now".
      The dream service should not be stopped after device rotated.
Change-Id: I454f75d06c4539258eeafd8794a3cb03e0276163