voice/video information tones

This is a copy of https://github.com/vector-im/element-android/issues/1687 as this is per client issue and no reaction from the core team has been received so far.

0. This is not only riotX issue, but all riot clients, including riot-ios and riot-web, but I'm not sure if 3 separate issues should be opened in each project.

1. First of all, when call is placed, the caller must hear beeps (ring.ogg) when and only when callee's phone has received incoming call notification and start ringing (vibrating / showing notification silently). There must be exactly the same principle as in GSM and SIP: if you hear beep it means the callee's phone is ringing.
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https://en.wikipedia.org/wiki/Ringing_tone :
> destination terminal is alerting the receiving party

Yes, I know that all modern IMs completely ignore this rule, cause all they suck.

2. There should be another sound to show caller that we're trying to initiate the call. Sometimes I hear repeating tone before ringing tone, it's similar to this one, that I've tried to find manually:
`for i in {1..5}; do beep -f 330 -l 80 -n -f 440 -l 80 -n -f 550 -l 80 -n -f 400 -l 80; sleep 1.5s; done`
I don't know what it means, how it's called, neither can't found any information about it, but I always treated it as a call initiation notification.

3. Unlike SIP and GSM, establishing p2p connection (call connection phase) may take significant time or fail. There must be special signal for this state, the silence is unacceptable here.