libaudioflinger: fix crash in LPA effects due to multiple tracks

- In gapless playback, the next track is created before the previous track
  is cleared. During this time, the effect chain is associated with the
  previous track
- If the next track is LPA, that track is not able to acquire the effect chain since
  the effect chain is associated with the previous track. This results in a failure
  in the creation of LPA track and hence a crash.

Change-Id: I8e3756f61fdb4401757317a92991888a369a4a6a
(cherry picked from commit d342a699a580d8ffe94416c59e1dc2c5321b8c4e)