"isPlaying" should never block, acquiring the mutex here is unnecessary,...

"isPlaying" should never block, acquiring the mutex here is unnecessary, rather than investing in a separate mutex guarding just "mFlags" we accept returning "true" from isPlaying one too many times.

Change-Id: I597577cbb3ad1afd3cb3ca91979ef24cc1a44658
related-to-bug: 2518319