Make DreamService detach on DreamActivity onDestroy

Currently DreamService.onDestroy is called whenever the DreamActivity is being destroyed. This causes issues with the lifecycle of the dream
window, because the DreamService is destroyed before
onDetachedFromWindow is called.

In this CL, we change the implementation of onActivityDestroyed to
detach the DreamService instead of destroy it immediately. This is a
more gentle way to achieve the same result.

Bug: 243385408
Bug: 266668856
Bug: 270913936
Test: atest DreamServiceManagerTests
Test: atest DreamServiceTests
Test: atest DreamOverlayTest
Change-Id: Iadb7880faabf54f70a64c76c6e3e267fde9c42dd