AudioMixer: Cleanups

- remove unused ARRAY_SIZE;

- fix indentation for TIMESTRETCH in setParameter;

- move trackNames method implementation and BLOCKSIZE constant
  into .cpp file;

- whitespace fixes.

Test: make
Merged-In: Idc5da55c8c3af0d59b9a7a12d081747b0a99dafc
Change-Id: Idc5da55c8c3af0d59b9a7a12d081747b0a99dafc