Show results immediately if user hits "back" key after finishing gameplay

[peppy]

Closes #18089.

I've gone ahead and matched the osu!stable behaviour for this, as it seems like it's what people are used to and they will settle for no less.

[bdach]

Two things:

First one, minor. With this change, do we still need the following code? It's not being removed in this pull.

osu/osu.Game/Screens/Play/Player.cs

Lines 570 to 580 in d2044fc

	// there is a chance that an exit request occurs after the transition to results has already started.
	// even in such a case, the user has shown intent, so forcefully return to this screen (to proceed with the upwards exit process).
	if (!this.IsCurrentScreen())
	{
	ValidForResume = false;
	// in the potential case that this instance has already been exited, this is required to avoid a crash.
	if (this.GetChildScreen() != null)
	this.MakeCurrent();
	return;
	}

I can't actually hit this path but it seems like it could be removable (just do nothing if screen isn't current).

Second, probably more important. If a storyboard has an outro, exiting during outro will not progress to results due to:

osu/osu.Game/Screens/Play/Player.cs

Lines 737 to 743 in d2044fc

	// If the current beatmap has a storyboard, this method will be called again on storyboard completion.
	// Alternatively, the user may press the outro skip button, forcing immediate display of the results screen.
	if (storyboardStillRunning)
	{
	skipOutroOverlay.Show();
	return;
	}

I haven't checked what stable does but the fact that skipping outro will get you to results but exiting kicks to song select feels quite bad regardless.

[peppy]

With this change, do we still need the following code?

Likely not. I noticed this but forgot to check whether it can be removed.

If a storyboard has an outro, exiting during outro will not progress to results due to..

Should definitely use the new behaviour instead.

[akiya-nagatsuka]

Sorry for a nitpick:
also need to add /// <returns> doc like for PerformExit?

[peppy]

I can't actually hit this path but it seems like it could be removable (just do nothing if screen isn't current).

It can be removed, but requires some (probably fine) changes to the restart process, which may add a bit of mental burden to the review process (see d174a6c).

Second, probably more important. If a storyboard has an outro, exiting during outro will not progress to results

Have fixed this, a very easy fix luckily. It even had test coverage.

[bdach]

I think this is okay, but leaning heavily on tests passing here...