Work around bug in offload driver

There is a bug in the offload driver that causes the last offload buffer(s)
to be dropped unless the device is on power or holding a wake lock.
To avoid truncated playback, we now hold a wake lock during the drain phase
of offloaded playback.

Bug: 19928717
Change-Id: I8df22e965ec791448aa5d9b74e743f48ef886fc4