Backport Android API-36 support.

[jpobst]

Context: #9914

Backport of #9914, et al to bring support for API-36 to .NET 9 (net9.0-android36.0).

Note our repository does not currently have build system support for having multiple stable API platform versions in a single .NET version. To work around this, this commit treats API-36 as an "unstable" API level for the context of our build system. However it removes the need for users to use $(EnablePreviewFeatures) and thus is a stable platform version to our users.

If we actually updated $(AndroidLatestStableApiLevel) to 36 it would have (at least) the following issues:

• We would no longer build or ship API-35 for .NET 9
• net9.0-android would switch from net9.0-android35.0 to net9.0-android36.0 which we do not want. Once shipped, net9.0-android should not change meaning.

Proper multi-stable support would require expanding $(AndroidLatestStableApiLevel) to something akin to $(AndroidMinimumStableApiLevel)/$(AndroidMaximumStableApiLevel) or @(AndroidStableApiLevels). We have chosen not to invest in this work at this time.

[jonpryor]

Shouldn't src/Mono.Android/PublicAPI/API-36/PublicAPI.Unshipped.txt be empty, as API-36 is "final"?

Have we run any API comparison between .NET 10's Mono.Android.dll and this new .NET 9 API-36 Mono.Android.dll? I assume they should be identical.

[jpobst]

Shouldn't src/Mono.Android/PublicAPI/API-36/PublicAPI.Unshipped.txt be empty, as API-36 is "final"?

I guess it could be. We generally do it when we ship in Nov, so there is a pretty long lag time between when we mark the Android API "stable" and when we the .NET is "stable". However, I guess we won't have that lag time for backporting. The downside is moving it here but not in main means the branches diverge a bit.

Ultimately it probably doesn't matter too much either way. From a tooling perspective there is no difference between "Shipped" and "Unshipped" files, and I doubt we will be adding new API to .NET 9 in the future.

Have we run any API comparison between .NET 10's Mono.Android.dll and this new .NET 9 API-36 Mono.Android.dll? I assume they should be identical.

Not explicitly, but the PublicAPI.[Un]Shipped files were simply copied from main unchanged, so if there were any differences PublicAPI would be complaining.