Rework NuPlayer::getStats() synchronization

undo earlier change moving this to the handler thread, instead using
a mutex to protect the contended resources: mAudioDecoder and
mVideoDecoder.

Bug: 110855771
Bug: 78365291
Bug: 111910846
Test: playback with persist.debug.sfstats=={0,1}
Change-Id: If8822dba9dae6ac0698b142166604371cd6742b2