Context: https://developer.android.com/about/versions/12

API-31 has been enumified (8843808, cf2a39b, cf2a39b).
Time to declare it stable!

Set API-S as API-31, `$(TargetFrameworkVersion)`=v12.0,
`$(AndroidPackVersion)`=31.0.100, and declare as stable.

Update unit test files so `$(TargetFrameworkVersion)`= v12.0 (API-31)
*or* `$(TargetFrameworkVersion)` isn't needed, a'la 8140991.

Rename `src/Mono.Android/Profiles/api-S.params.txt` to
`api-31.params.txt`, to match the stable API-level ID.

Updated `tests/api-compatibility/acceptable-breakages-vReference.txt`
because the default API compare will now be against `API-31`, not
`API-30`, and we deliberately removed the `[Category]` custom attribute
(e5a4bee), remove `[DataContractAttribute]` (101fea2), and fix the
value of `Android.OS.BuildVersionCodes.R` (1b1ec09).

Support `$(AndroidUseLatestPlatformSdk)` in binding projects: once
API-31 is stable, binding projects with
`$(TargetFrameworkVersion)`=v11.0 and
`$(AndroidUseLatestPlatformSdk)`=true fail to build with:

	Task ResolveNuGetPackageAssets
	...
	Microsoft.NuGet.targets(198,5): Your project does not reference "MonoAndroid,Version=v12.0" framework. Add a reference to "MonoAndroid,Version=v12.0" in the "TargetFrameworks" property of your project file and then re-run NuGet restore.

What happened is that `$(TargetFrameworkVersion)` was changed to 12.0
during `Build`, but not during `Restore`. Thus NuGet restored with
`MonoAndroid,Version=v11.0` instead of `MonoAndroid,Version=v12.0`.

I found that we have this setup for other project types, but not
binding projects:

	<!--
	  NOTE:
	  This target runs during Restore, and there is no $(RestoreDependsOn) property.
	  There appears to be no other way to do this other than use BeforeTargets.
	-->
	<Target Name="_SetLatestTargetFrameworkVersionForPackageReference"
	    Condition=" '$(AndroidUseLatestPlatformSdk)' == 'True' "
	    BeforeTargets="_GetRestoreTargetFrameworksOutput"
	    DependsOnTargets="_SetLatestTargetFrameworkVersion">
	</Target>

If we move this target to `Xamarin.Android.Legacy.targets`, it will be
imported by binding projects.

We also get the added benefit of it not being in .NET 6.
`$(AndroidUseLatestPlatformSdk)` is not supported in .NET 6, so we
should not have this MSBuild target there.

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>