ActiveAE: add advanced setting for forcing multichannel layout

Anthem Statement D2 audio processor only supports 2.0 and 5.1 HDMI
playback properly, causing completely silent output if playback of e.g.
a 4.0 audio stream is being done.

Add "audio.fixedmultichannellayout" advanced setting for forcing the use
of the user-specified layout in audio settings for multichannel streams
in case the user has equipment that does not support all intermediate
layouts properly.
Stereo streams will continue to be played back as stereo and no
resampling is done.

Reported by Grant Warecki (gjwAudio).

@fritsch, @FernetMenta, WDYT? This basically becomes a oneliner in ApplySettingsToFormat() so IMHO it does not pollute the code too badly for a fix for obscure devices...