audioflinger: keep wakelock during offload playback

Add a system property ro.audio.offload_wakelock to select
if a wakelock should be kept during offload playback while waiting for
write completion callback.
The default is yes, keep the wakelock.

On some platforms, the cost of entering or exiting suspend is so high
that is it better to not release a wakelock at all rather than releasing
it periodically.

Bug: 26208297
Change-Id: Ib41c7e4ed9c8c0c063363eb9fe99a2ecc81b521b