post processing: add volume command support

By design of the effect offloading mechanism, offloaded implementations
must have the same behavior as non offloaded implementations.
As the non offloaded effects (EQ, Bass boost, Virtualizer ...) from the
AOSP effect bundle implement the EFFECT_FLAG_VOLUME_CTRL flag and
behavior, offloaded effects should.

In order for volume control to work, EFFECT_CMD_SET_VOLUME must be
implemented and return the received volume in the reply data.

Bug: 37281217
Test: Music playback in offload and non offload mode with and without EQ.
Change-Id: I2425f4dce95c8ab877db19cefbaa6226895b4015