audio: Fix for race conditions in direct audio track

-Issue: Mediaserver crash is observed in direct track functions.
-Rootcause: Between write and flush functions, write and destructor
effectpool buffers are accessed without any synchronization that
 might lead to operations on null buffers and cause crash.
-Fix: Use mEffectLock to synchronize access of effectpool buffers.

Change-Id: I5982e31fd6d292a9fe28dfee4cf75ebfe0ade5e6
CRs-Fixed: 515107
(cherry picked from commit b14c28f0d3b51ee989640aa46f3addcfabccb6be)
(cherry picked from commit 782f8512023facb1b655fb636bdcbd19af21b493)
(cherry picked from commit 1601287d6a06e11d782a821791bf1f5a9dcbd188)