ClientProvider found its own client: from the widget API, or by restoring
or creating a session. A host that embeds Element Call already has one,
and owns the user's session, so accept it as a prop and skip all of that.
A supplied client seeds the state synchronously, since there is no session
of ours to restore and so nothing to wait for.
Also guard the broadcast that shuts down other instances of the app. It
protects Element Call's own session and crypto stores, which is why it was
already skipped in widget mode — a host's client has the same property, so
without this an embedded Element Call would close down the user's other
tabs on mount.
Adds the first tests for ClientContext, covering both
Valere