The code was only entering the blocked state if the user was speaking,
which often won't be the case when another person starts speaking because
we'll have pressed the button but not got the ack back from the server
yet. Add the transmitblocked flag instead so we don't enter that state
again if we've already decided we've been blocked.
We were also starting with blocked = false and so resetting it when it
shouldn't have been reset.
Also requires https://github.com/matrix-org/matrix-js-sdk/pull/2502
2b92bf3694