Power connect/disconnect notification support
Make Safe headset volume user configurable
AudioService: Restore volumes after boot and when a headset is plugged

The correct volume levels for <device>_speaker and <device>_headset are not
restored after booting and when a headset device is plugged in or out.

This fixes a glitch where the first volume change event causes the volume to
be reset to the last stored value instead of increasing/decreasting it by
one step.