Optimise MatrixRTC sender key distribution for case of single member

[hughns]

Based on top of https://github.com/matrix-org/matrix-js-sdk/pull/4453.

Optimises two scenarios:

• user is only member of call, then one other member joins => the user can immediately rotate the key and start using it.
• there were two members in the call, then the other member left => the user can immediately rotate their key and start using it.

The "doesn't re-send key immediately" test is superseded by the other test cases.

Checklist

• [x] Tests written for new code (and old code if feasible).
• [x] New or updated public/exported symbols have accurate TSDoc documentation.
• [x] Linter and other CI checks pass.
• [x] Sign-off given on the changes (see CONTRIBUTING.md).