Don't poke user activity when dream ends

The dream may end for various reasons, including some that do not
represent user activity, such as app crashes or the app being updated.
In these cases, the user activity should not be poked, as this would
reset the inattentive sleep timer.

Bug: 215559033
Test: atest PowerManagerServiceTest
Test: Kill dream app, observe `dumpsys power` lastUserActivityTime
Test: Kill dream app, verify inattentive sleep timer not affected
Change-Id: If9b61cab3d5e21b89f3c0ab7fe541d46bc865f97