When a DreamActivity is destroyed, the devices remains in a dream state.
This makes it look like the DreamActivity is stuck in the background but
it is actually destroyed and DreamService does not allow another Dream
to take its place.
Bug: 242368903
Bug: 241569844
Test: Manually tested by checking that when a DreamActivity is destroyed
the device wakes up from dreaming.
Change-Id: Idcb38f2332f3a8dea0add78a4ca506877aeca8f7
feb3aa71f9