Fix ringing chirp on loading
We have a heap of logic to do the right thing when a call event
arrives, eg. wait until the client is ready so we can see if there's
already been a hangup event before saying there's an incoming call.
Unfortunately it only waited until the client was prepared, not
until it was syncing, so any events that arrived from the server
in the catchup sync bypassed this logic altogether.
This was probably broken back when we introduced the sync accumulator,
since before then, "PREPARED" meant, "done initialsync" rather than
"loaded fake initialsync from storage".
Fixes https://github.com/vector-im/riot-web/issues/3572
