libaudioprocessing: Extract vendor-available part of AudioMixer

Split AudioMixer into the base part which doesn't rely on
the framework and can be used in vendor code and the derived part
which is intended to be used by Audioflinger.

Test: A/B compare output from test scripts, manual testing on the phone
Merged-In: I24c390f67f20baa8109902099359ca6e34eebcfd
Change-Id: I24c390f67f20baa8109902099359ca6e34eebcfd