CHG: [droid] refactor wake lock; follow our screensaver

This refactors the wake lock to release it when our screensaver goes active.

This allows:
- Control on the wake lock via our screensaver settings
- Devices to go to sleep when we are inactive (e.g. Amazon FTV and tablets)

/cc @Montellese @davilla @t-nelson
We could, but wouldn't that "pervert" announcements, which is imo a functional thing, not a technical one?
Not even speaking about loosing cycles at each and every announcement by going thru an additional, new listener, of course...