Fix MatrixRTC sender key wrapping

[hughns]

Checklist

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

[toger5]

This makes a lot of sense if currentEncryptionKeyIndex is sth that is available to us anyways.

[hughns]

I found since that we actually need to make the maximum key index configurable depending on which MatrixRTC backend is being used.

MSC4143 will actually mandate 256 indices, so this can stay for now.