VoIP: relay candidates still don't show up from the mobile side

**Describe the bug**
Copying this over from https://github.com/vector-im/element-android/issues/2818#issuecomment-787783062
So with 1.1.0 out, according to my tests it still doesn't support TURN servers and WebRTC relay candidates.

I could only make calls with using STUN servers and UDP hole punching when behind NAT or firewalled IPv6 connections: tracked calls from mobile to browser and vice versa with `about:webrtc` in Firefox/Librewolf, no relay offers from the mobile side, while I had them on the browser side.

Is this intentional?

**To Reproduce**
Steps to reproduce the behavior:
Just start a voice call on mobile and see what candidates show up in WebRTC from the mobile client.

**Expected behavior**
With a TURN server correctly set up with credentials (that works in element-web), relay candidates should show up from the mobile client as well.

**Smartphone (please complete the following information):**
- Device: ASUS ZenFone 6, my brother's Poco F2, and another acquaintance's phone of which brand I don't know.
- OS: Android 11, but IOS devices seem to have the same issue according to my tests.

**Additional context**
- I have 1.1.0 from F-Droid, my brother has the same from the play store.
- Homeserver: ksol.io

I use my own coturn server on FreeBSD in a separate jail, that has some issues unfortunately due to it not properly supporting it and/or being buggy, which I plan to fix by contributing to its codebase when I'll have the time for it. That said, as mentioned before, relayed calls with coturn work between two browsers (without a STUN server, and setting the two browsers to use use relay-only candidates). I would expect the same from the mobile clients.