Consolidate @(AndroidLibrary) item groups for .NET 6

[jonathanpeppers]

As discussed a couple things can improve upon fcd7cf8:

1. We should support @(AndroidLibrary) in "legacy" Xamarin.Android and document it. It seems like a convenience that would not be to difficult to support.
2. We should "hide" the old item groups in .NET 6 (via @(AvailableItemName)) so they don't appear in the dropdown in IDEs.
3. @(AndroidNativeLibrary) should remain distinct from @(AndroidLibrary) as the %(Bind) and %(Abi) metadata do not really make sense for both types of libraries.

[jonpryor]

"Related" from our discussions earlier today: we need to overhaul the XA4301 error message emitted, currently:

Native library '{0}' will not be bundled because it has an unsupported ABI.

This message is not actionable, and thus in the proposed workflow:

1. User adds libexample.so file to the directory containing Foo.csproj
2. User builds Foo.csproj
3. Build quickly fails with XA4301, as we can't infer the ABI for libexample.so.

We thus need to update XA4301 with an appropriate fix, to either

1. Edit Foo.csproj to add libexample.so to an @(AndroidNativeLibrary) item group, and set %(Abi), or
2. Move libexample.so into an e.g. libs/ABI directory.

[jonathanpeppers]

I filed a different issue for the error message: #5358