audioflinger: 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.

CRs-Fixed: 515107
cherrypicked from Ib071361c65b390f9486c0a080a9391f176392b30

Change-Id: Id9e14a054d699ec803e23619ef475696b0021d4f