Incoming call bricks ongoing call audio

@earthworm Even without "call waiting" implemented this must not happen and would be a separate issue. Making a new bug report from #209

That one talks about implementing call waiting. But for now, if I get another call come in, it nukes the audio of the ongoing call.

I agree with @guidog. Current working theory: When the second call gets hung up libcall-ui sets the audio mode back to "default".

If so, then running callaudiocli -m 1 with the initial call still "active" should bring audio back.

I just managed to test this, and that command does get me the call audio back, so theory confirmed.

I also noticed in testing this ...

1. Call some number with a long queue and hold music from Pinephone
2. Call myself from the office phone

The office phone hears "The number you have called cannot be connected", rather than anything about them being engaged.

The Pinephone beeps for the incoming call, then the audio cuts off for the ongoing call. The callaudiocli command above brings it back.