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.
QS: Link the screen timeout tile to the settings

This commit simply makes long-pressing the back of the sleep tile (the
screen timeout selection part of the tile) start the appropriate
settings activity (Display settings). This matches the behaviour of
most other QS tiles where there are additional settings screens.

Change-Id: Ia36764f1a606cd3166a11516fe3709ce9bffd125