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
Change-Id: I24c390f67f20baa8109902099359ca6e34eebcfd