On Android, users were unable to join or create multiplayer rooms. The
root cause of that was that the both the wait and set of the
`ManualResetEvent` in `getRoomUsers` occurred on the same thread, which
created a chicken-and-egg situation - the set could not proceed until
the wait had actually completed.
Resolve by substituting the `ManualResetEvent` for a
`TaskCompletionSource` to achieve a promise-style task, which the
previous code was a crude approximation of anyway.
Closes#11385.
It needs to be explicitly stated that the users in this list are related
to the *joined* room. Especially since it's sharing its variable name
with `SpectatorStreamingClient` where it has the opposite meaning (is a
list of *globally* playing players).
Fixes test failures due to not allowing to do so, therefore inverting
execution order in some cases - for example, calling
JoinRoom(room);
LeaveRoom();
on the update thread would invert execution order due to the first being
unscheduled but the second being scheduled.