Respect volume setting of iOS during video call

Riot does not respect the volume settings of the iOS Device during video calls.

Steps to reproduce:
1. Talk to someone via the video chat
2. change volume

Expected:
The volume changes

Currently:
No change in volume.

Start by reproducing the video-call scenario on iOS and inspect the audio handling entry points used during calls. Verify how device volume changes are observed and applied while a call is active; done means changing the device volume changes the call audio as expected.

Written by the indexing model from the issue text.