nuplayer: acquire wakelock during offload pause timeout

Hold a wakelock while paused in audio offload mode, until audio
teardown event is received, or pause is canceled.

Since ARM cores are sleeping, the delayed AMessage will not be
received, and the audio output driver will not be closed.
This blocks XO shutdown.

Bug: 19297092
Change-Id: I3d11fef633e3f2783f4aa31e71285e2fa09d234c