Do not try to access service in DreamActivityCallbacks after destroy.

This changelist ensures that DreamActivityCallbacks takes no action on
the service once it's cleared in onActivityDestroyed.

Test: atest DreamServiceTest#testCallbackInvalidAfterDestroy
Fixes: 394086457
Flag: EXEMPT bugfix
Change-Id: If2f7d6d9c908628813b00c775d6c6524ffbec346