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
Why don't we release the wakelock anymore if someone puts the app into the background?