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.

Change-Id: I6a483af34fd6d9dd706c6979202bf0cf26d80854