Stereo downmixer supports generic configurations. Fix 7.1 downmix

Added a generic downmixer to stereo function to handle the
  multichannel configurations not supported by the dedicated
  downmix functions.
  It first verifies the mask is supported, and then derives
  channel indices for the downmix.
Added support for forcing the generic downmixer to be always
  used instead of the format-specific functions (define
  DOWNMIX_ALWAYS_USE_GENERIC_DOWNMIXER to force it).

Fixed 7.1 downmix function where handling of accumulate vs
  overwrite was inversed.

Bug 4280902

Change-Id: I8259b32c4e90f76ef4dcd803592fc71df4ae90c5