audio-only,lastn: move audio-only and last N handling to standalone features
This refactors all handling of audio-only and last N to 2 features in preparation
for "low bandwidth mode".
The main motivation to do this is that lastN is a "global" setting so it helps
to have all processing for it in a single place.
[RN] Implement full screen mode while in a conference
The implementation varies across platforms, with the same goal: allow the app to
use the entire screen real state while in a conference.
On Android we use immersive mode, which will hide the status and navigation bars.
https://developer.android.com/training/system-ui/immersive.html
On iOS the status bar is hidden, with a slide effect.