Missing midi demuxer & failing midi decoder

Coming from https://forum.videolan.org/viewtopic.php?f=35&t=151539

We don't ship the midi demuxer (https://code.videolan.org/videolan/vlc-android/-/blob/80b396d318e5f9686f4087377ea6f0fab93664cb/buildsystem/compile-libvlc.sh#L389)

The decoder fails because there is no soundfile to be opened. We most likely ought to use mediacodec for this, but the audio support for media codec is still experimental

Shipping the demuxer is trivial, but I'm not sure it's really worth something until we can play the files