hal: Disable gapless offload playback by default.

-Set the property-audio.offload.gapless.enabled to true to enable
 gapless playback by default.
-Set mixer control to the driver to decide if gapless playback needs
 to be used or not
-If gapless playback is not enabled then partial drain from HAL is
 treated as full drain.

Change-Id: Id7f4967fe528bab9097c3f609058276ec02174e1