[VoIP] Visual feedback of recipient device starting to ring

### Your use case

#### What would you like to do?

I want to see a visual change in the call screen when the recipient's device receives the call notification and starts ringing.

#### Why would you like to do it?

Currently, we have no way to know if the app is still waiting to connect to the recipient device or if it has already reached it and is waiting for the recipient to pick the call.

This can be incredibly time wasting in situations such as when the recipient is not connected to the internet at all. You don't know if their phone is ringing and they are taking their time to pick up, or if their device is just not connected.

#### How would you like to achieve it?

Example: `Waiting...` -> `Ringing`

### Have you considered any alternatives?

I haven't looked, but maybe a quick survey of other VoIP-capable apps could be done to figure out the best UI/UX here.

### Additional context

https://github.com/vector-im/element-android/issues/1687#issuecomment-657946223
https://github.com/vector-im/element-android/issues/3221#issuecomment-835387704