Beyond isolation, this stuff is also used to test (de-)serialisation.
Much-of-a-muchness in this case since it isn't too thoroughly tested,
but best do it anyway.
Removes storage of `selectedBeatmap` that was referenced through
multiple class-level methods.
To expound a bit, this structure felt better (or otherwise passing
`APIBeatmap` through methods) alongside removal of the `#nullable
disable`, otherwise each method would check `selectedBeatmap != null`.
It proved to be too difficult to deal with the flow that clears user
states on stopping the watching of global presence updates. It's not
helped in the least that friends are updated via the API, so there's a
third flow to consider (and the timings therein - both server-spectator
and friends are updated concurrently).
Simplest is to separate the friends flow, though this does mean some
logic and state duplication.