FIX: [droid] restore system volume on exit

XBMC forces the "media" volume on startup to his own, remembered value and leave it like that (+/- adjustments during the session) on exit.
This is somewhat problematic as the "media" volume also controls notifications. I often happen to watch videos muted, but then, on exit, my device is muted system-wise, including notifications.

I see 2 ways of solving this:
1) Ignore XBMC remembered value, get system volume on startup and go on as today
2) Restore initial system volume on exit/lost focus.

I implemented 2)