Element Call PiP (Picture in picture) in Element X

Presently, there's too many unknowns to implement PiP (picture in picture) in the Element Call/X integration in a low hanging way. We need to better understand:

- Which video feed PiP shows. Preference would be the active speaker but it's not clear if this will work out of the box with native PiP APIs, per platform.
- How to navigate between active calls and PiP. On iOS, it's likely we'll rely on modality. On Android, it's likely we'll rely on a full screen view with a back/PiP button, but embedded within the webview for implementation ease.

Therefore, to simplify the implementation for now, we'll:

- Use a full screen webview on both platforms
- Hide the share button when embedded. This is the only difference between the browser & embedded experience, which makes for easy design, build & maintenance.
- Expect users to use the big red 'leave call' button to leave calls.

Visual reference (ignore the finer details, it's just to visually aid the bullets above):

Then, when we can spend more time on PiP in future:

- We can work through the unknowns above.
- To get working implementations going, we can repurpose the 'share' button as a 'PiP' button in developer UI.
- After feeling out working implementations, we should expect to need to adjust the header layout in the SPA. This 'double handling' should ultimately be light in effort.