Closes https://github.com/ppy/osu-framework/issues/4619 (actually not a
framework issue; the framework correctly guards against this scenario,
see
4e29504384/osu.Framework/Screens/ScreenStack.cs (L277)).
Added the assertions to be very explicit about the nested stack's state
at this point. Both of those events can only be triggered if the stack
has a loaded screen (as far as I can tell), making this check
unnecessary in those cases.
Finishing an operation started via
`OngoingOperationTracker.BeginOperation()` was risky in cases where the
operation ended at a callback on another thread (which, in the case of
multiplayer, is *most* cases). In particular, if any consumer registered
a callback that mutates transforms when the operation ends, it would
result in crashes after the framework-side safety checks.
Rework `OngoingOperationTracker` into an always-present component
residing in the drawable hierarchy, and ensure that the
`operationInProgress` bindable is always updated on the update thread.
This way consumers don't have to add local schedules in multiple places.
