Add [SupportedOSPlatform] attributes to assemblies using ApiSince.

[jpobst]

Context: dotnet/android#5338

.NET 5 provides a new attribute [SupportedOSPlatform] to specify when an API is not available on all platforms or platform versions. This is used to build analyzers to give users warnings if they are trying to use an API when it will not be available on their target platform. This is fundamentally the same as our existing ApiSince, except tooling has actually been built to consume the information.

As such, we need generator support to put this information into Mono.Android.dll.

// Metadata.xml XPath method reference: path="/api/package[@name='android.app']/class[@name='Activity']/method[@name='dismissKeyboardShortcutsHelper' and count(parameter)=0]"
[global::System.Runtime.Versioning.SupportedOSPlatformAttribute ("android24.0")]
[Register ("dismissKeyboardShortcutsHelper", "()V", "", ApiSince = 24)]
public unsafe void DismissKeyboardShortcutsHelper ()
{
	const string __id = "dismissKeyboardShortcutsHelper.()V";
	try {
		_members.InstanceMethods.InvokeNonvirtualVoidMethod (__id, this, null);
	} finally {
	}
}

Some interesting notes:

• This attribute is only available in .NET 5+, so we include a local version for earlier frameworks so we can compile without needing to #ifdef every attribute.
• The attribute is marked as [Conditional ("NEVER")] so the attributes will not actually get compiled into the resulting pre-NET5.0 assembly.
• The attribute cannot be placed on interfaces or fields, so we aren't able to annotate them.
• Our minimum supported API for .NET 6 is 21, so we only write attributes for API added in versions newer than 21, as API added earlier are always available.

[jonpryor]

If we are defining this type, in a pre-.NET 5 context, shouldn't this attribute be internal, not public?

[jpobst]

Yep, making it public causes type conflicts when a pre-.NET 5 Mono.Android.dll is consumed by a .NET 5+ Android app. Marked internal.