Support for MSC3291: Muting in VoIP calls

[SimonBrandner]

The implementation (matrix-org/matrix-js-sdk#1812, matrix-org/matrix-react-sdk#6486) has been merged on Element Web, so it would be nice if Android supported this too. The implementation should be relatively easy

[SimonBrandner]

The implementation should be relatively easy

Thinking about this again EA would need to send only m.call.sdp_stream_metadata_changed to not tell other clients that they support MSC3077 (see
https://github.com/SimonBrandner/matrix-doc/blob/msc/sdp-metadata/proposals/3077-multi-stream-voip.md#backwards-compatibility)

[kittykat]

This applies both to audio only and video calls