Implement ringback tone

### Your use case

#### What would you like to do?
Referring to an older issue from [Element Classic](https://github.com/element-hq/element-android/issues/9101)

I would like to hear some kind of tone when placing a call, instead of silence. The silence makes the user think the call is not working when it indeed is.
#### Why would you like to do it?
Ideally, if it's possible to differentiate when the receiving user's devices have started ringing, I would like to see a "connecting" tone and then a different tone when the call is actually ringing on the other end. A good example is calls on Telegram, Facebook Messenger or pretty much any major IM app with calls.

While we're at it, might be good to have a busy tone as well when the other party declines the call.
#### How would you like to achieve it?
Either use a sound generator library or source some ready made sounds and play them on relevant parts of the call lifecycle.

I might take a stab at implementing it sometime but I don't know yet when I might have time for this.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

### Are you willing to provide a PR?

Yes

The issue names no files, tests, or call-lifecycle entry points. Start by locating the Android call flow and its existing audio handling, then determine how connecting, ringing, and declined-call states are represented; done means tones play at the appropriate stages without disrupting calls.

Written by the indexing model from the issue text.