Indicate screen off condition to audio framework.

Add a mechanism for AudioService to notify audio framework and audio HAL
that screen is turned on or off. This indication can be used by lower
audio layers to trigger low power audio playback mode.
Current implementaiton based on setParameters() method is experimental
and will be replaced by a different interface when ready.

Change-Id: Ie72ada0a8edec3d71b47681b9f51cb5bc839d45b