Fix SampleChannelBass reporting wrong state

[hwsmm]

This PR fixes SampleChannelBass reporting wrong state.

• Calling Stop no longer makes IsAlive false.
• Calling Play after disposal no longer makes enqueuedPlaybackStart true, so that Playing won't be true (this can be tested with TestDoesNotRestart because SampleChannel is disposed after the end of playback with Dispose SampleChannel in Sample if it is not disposed yet #6397).

Do not merge this yet. What should we do for looping samples? Stop no longer makes IsAlive false, but looping sample will be a zombie because it doesn't end. Should we make SampleChannels not resumable?

[smoogipoo]

Should we make SampleChannels not resumable?

How about we add a bool ManualFree { get; set; } to the interface? Opposite of BASS' auto-free because interfaces can't have default values (we really should be doing sample.GetChannel(new ChannelProperties { ... }) or similar, but...).

And then, every other case should auto-free when playback ends or Stop() is called.

I believe everywhere in osu! we operate under the assumption that channels are auto-freeing by default as I believe that's effectively how they work in BASS when they're HCHANNELs, so this could be considered a regression from when mixers were added (and they were made into HSTREAMs).