don't use SoftAAC for downmix

if no multichannel output is found android mixer will downmix

Change-Id: I5858a88033121cf9bdd926ff7d8bf5086c1aef0e
dedupe soft video codecs

Remove superfluous ffmpeg decoders for codecs that can be handled
by OMX.google

Change-Id: Ib3648ab85a1df096bc94b8809eaec8f80687984d