update the behavior of playback when screen is turned off
- previously
* all playback would stop when screen was turned off
* no media could begin while screen was in the off state
- now:
* if the current playlist item has a video track
- pause when the screen is turned off
- resume when the screen is turned back on
* if the current playlist item is audio-only
- allow playback to continue when the screen is turned off
- allow playback to begin while screen in in the off state
- for both cases:
* set a partial wakelock to prevent deep sleep
* remove the partial wakelock when screen is turned back on
additional tweaks:
* spacebar key toggles pause/resume
- useful with an air-mouse on a set-top box
