AE: add settings for ac3 transcode and ac3 upmix

It turned out that we can't autodetect if AC3 transcoding is desired. We always engaged it in case of no LPCM and enabled AC3. This does harm on the PI.  There we want AC3 only for AC3 content and no transcoding at all.

Also fixes slight regression: missing stereo upmix via AC3 transcoding for HDMI devices.