Set of fixes for Bluetooth volume control issues:
* split OS volume from A2DP device volume;
* added translations for additional popup required
to indicate what volume is being set;
* fixed issue that on some devices setting the lowest
volume level would set the highest in reality;
* fixed Bluetooth auto-turnoff functionality;
* minor code cleanup.
Added volume control feedback during music playback over Bluetooth.
Changed volume buttons handler logic in service audio to use actions
instead of messages.
