Fix issue #6634325: View.setKeepScreenOn and...

...MediaPlayer.setScreenOnWhilePlaying seem broken

We need to correctly clear the keep screen on flag when the view
hierarchy request is gone...  and to do that, we need to keep the
actual state of the flag requested by the app.  Also when the app
changes its state, we need to compute the proper value based on
both the app request and any requests in the view hierarchy.

Bug: 6634325
Change-Id: I060e9a34a10faffbaa77c06098cf21298bb4969f