[Mono.Android] Fix WindowManagerLayoutParams.SystemUiVisibility enumification

[jpobst]

Fixes #4290

The following API were incorrectly enumified as StatusBarVisibility instead of SystemUiFlags:

• android.views.View.[get|set]SystemUiVisibility (int) (source)
• android.views.WindowManager.LayoutParams.SystemUiVisibility (source)

As these are properties, we cannot create an overload. Instead create a new property called SystemUiFlags and obsolete the incorrect property. Additionally, this whole thing was deprecated in API-30, so make sure we reflect that on the new method. It's still worth fixing this, as people will be using this for pre-API-30 device support for years to come.

API Reference Results:

WindowManagerLayoutParams

View

[jonpryor]

Given that we'll eventually do a net7.0-android34.0 release, requiring cherry-picks to a release/7.0* branch, should this instead be:

#if NET && ANDROID_34

[jpobst]

Guarded just in case. However, I think we should expect net7.0-android34.0 API may not exactly match net8.0-android34.0 and we shouldn't backport these fixes.

I don't suspect a user upgrading to .NET 8, using new API, and then wanting to downgrade their app to .NET 7 is a common scenario.

[jonpryor]

@jpobst: I'm more concerned about the opposite: customer starts with net7.0-android34.0 -- probably released in August or September! -- then migrating to net8.0-android34.0 in November.

Then there is ABI compatibility: libraries built against net7.0-android34.0 should work as-is in a net8.0-android34.0 app. Will it?

[jpobst]

That should not be an issue with this PR, as it only adds API to newer versions, but it is good to keep that in mind in general.

[jonpryor]

Re-reading this, and I feel like this is incomplete?

The PR description states:

WindowManagerLayoutParams.SystemUiVisibiliy (source) was incorrectly enumified as StatusBarVisibility instead of SystemUiFlags.

However, the source link is to View.setSystemUiVisibility(), while it mentions WindowManagerLayoutParams.SystemUiVisibiliy. These are not the same.

Similarly, the PR contents adds a WindowManagerLayoutParams.SystemUiFlags property, but issue #4290 was about View.SystemUiVisibiliy.

Shouldn't this change hit both WindowManagerLayoutParams and View?

[jonpryor]

Fixes: https://github.com/xamarin/xamarin-android/issues/4290

The [`View.getSystemUiVisibility()` method][0], bound as the
[`View.SystemUiVisibility` property][1], and the
[`WindowManager.LayoutParams.systemUiVisibility` field][2], bound as
the [`WindowManagerLayoutParams.SystemUiVisibility` property][3],
were incorrectly enumified to use the `StatusBarVisibility` enum when
they should have used the `SystemUiFlags` enum.

We cannot change the existing members, as that would be an ABI and
API break.  Furthermore, as these are properties, we cannot overload
them either.

"Fix" this by:

 1. Marking the existing properties as `[Obsolete]`, and
 2. Introduce *new* `SystemUiFlags` properties.

Additionally, these members were deprecated in API-30, so make sure
we reflect that on the new method.  It's still worth fixing this, as
people will be using this for pre-API-30 device support for years to
come.

These changes will only be present in API-34+.
The resulting API is:

	partial class View {
	    // Original property
	    [Obsolete ("This property has an incorrect enumeration type. Use the SystemUiFlags property instead.")]
	    public StatusBarVisibility SystemUiVisibility {get; set;}

	    // New hawtness
	    [ObsoletedOSPlatform ("android30.0", "…")]
	    public SystemUiFlags SystemUiFlags {get; set;}
	}
	partial class WindowManagerLayoutParams {
	    // Original property
	    [Obsolete ("This property has an incorrect enumeration type. Use the SystemUiFlags property instead.")]
	    public StatusBarVisibility SystemUiVisibility {get; set;}

	    // New hawtness
	    [ObsoletedOSPlatform ("android30.0", "…")]
	    public SystemUiFlags SystemUiFlags {get; set;}
	}

[0]: https://developer.android.com/reference/android/view/View#getSystemUiVisibility()
[1]: https://learn.microsoft.com/en-us/dotnet/api/android.views.view.systemuivisibility?view=xamarin-android-sdk-12
[2]: https://developer.android.com/reference/android/view/WindowManager.LayoutParams#systemUiVisibility
[3]: https://learn.microsoft.com/en-us/dotnet/api/android.views.windowmanagerlayoutparams.systemuivisibility?view=xamarin-android-sdk-12