MatrixRTCSessionManager is tracking encryption keys even when not needed

Currently the js-sdk always tracks the encryption keys even if the client never does anything with media.

The impact at the moment is two fold:

- Element Web is doing unnecessary duplicate tracking of encryption keys
- It makes for confusing log lines like in https://github.com/element-hq/element-call/issues/2561

Proposal: we make it so that `MatrixRTCSessionManager` doesn't track encryption keys by default, but instead the client has to call something like `MatrixRTCSessionManager.startTrackingEncryptionKeys()` which EC would use.

Start by locating MatrixRTCSessionManager and tracing where encryption-key tracking begins, then inspect how Element Web and Element Call use the session manager. Done means tracking is opt-in through a client-facing start method, with no tracking or confusing log lines for clients that do not use media.

Written by the indexing model from the issue text.