Development: Add choices to 'Stay Awake' preference

* Per a request on Google+ for Google Engineers to add to M, the stay awake values
    can now be never, while usb debugging, and always.
* Keep them pixels awake.

Change-Id: I7f43200bab0f000dc7547cc3142c4cacda2f4732
Signed-off-by: Brandon McAnsh <brandon.mcansh@gmail.com>