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
It was actually a JSON-RPC thing but everyone has started abusing it anyway...

I'm not saying that it's ideal but I prefer not to clutter CApplication even more. Your call in the end.