powermgmt: adjust sleep timer behavior
Responding to the bug report posted by iPodVT:
https://forums.rockbox.org/index.php/topic,55180.msg255292
- An active sleep timer that runs out will now cause the
player to shut down regardless of playback state
- When playback state is paused or stopped, once the
idle timer has run out, player will shut down, regardless
of any running sleep timer
Change-Id: I33de682a63ed1db76174eb2394ef5568f37cc677