[Xamarin.Android.Build.Tasks] Add support for 'AndroidEnableRestrictToAttributes'.

[jpobst]

Context: dotnet/java-interop#1081
Context: dotnet/java-interop#1094

Adds support for emitting [Obsolete] warnings for API marked with androidx.annotation.RestrictTo. These warnings inform the user that the package owner does not consider an API to be "public". These warnings have a custom warning code (XAOBS001) that can be suppressed if desired.

This is enabled by default with the value obsolete, but can be disabled by setting $(AndroidEnableRestrictToAttributes) to disable.

<AndroidEnableRestrictToAttributes>disable</AndroidEnableRestrictToAttributes>

[jonpryor]

Should we also add:

If you would instead prefer to *remove* members that have the `@RestrictTo` annotation *or* are in non-exported Java packages, you can use [Transform files](https://learn.microsoft.com/xamarin/android/platform/binding-java-library/customizing-bindings/java-bindings-metadata#metadataxml-transform-file) to prevent these types from being bound:

            <attr path="//class[@annotated-visibility]"
                name="visibility">kotlin-internal</attr>
            <attr path="//interface[@annotated-visibility]"
                name="visibility">kotlin-internal</attr>

[jonpryor]

…formatted properly to use XML block formatting, as appropriate.

[jpobst]

Updated using <remove-node>.

[jonpryor]

We're gonna change the boolean MSBuild property to an "enum" property.

[jonpryor]

WIP commit message (will need updating for enum semantics):

Context: https://github.com/xamarin/java.interop/issues/1081
Context: https://github.com/xamarin/java.interop/commit/b274a67f38e2f4012ab8414cba2c778f53f0b13c
Context: https://github.com/xamarin/AndroidX/issues/690

Android libraries may use the [`androidx.annotation.RestrictTo`][0]
annotation to mark a `public` Java type as "not public":

	// Java
	@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
	public class DrawableWrapper extends Drawable {
	}

Unfortunately, .NET Android didn't know about this annotation, so all
such annotated types were bound as *`public`* types:

	// C# binding
	public class DrawableWrapper : Drawable {
	}

This is a problem because Google doesn't maintain API compatibility
for types with the `@RestrictTo` annotation.  This can result in
undesirable API breakage; see also xamarin/AndroidX#690.

xamarin/java.interop#b274a67f updated `class-parse` to know about the
`@RestrictTo` annotation; when present, an `//*/@annotated-visibility`
attribute is present within `api.xml`:

	<class
	    name="DrawableWrapper"
	    …
	    annotated-visibility="LIBRARY_GROUP_PREFIX"
	/>

xamarin/java.interop#b274a67f also updated `generator` to support a
`generator --lang-features=restrict-to-attributes`; when present,
types with an `//*/@annotated-visibility` attribute will be marked
as `[Obsolete]` (*not* removed!), in order to maintain API
compatibility with existing ("broken") bindings, so that customers
can migrate away from these types:

	// C# binding
	[Obsolete
	    "While this type is 'public', Google considers it internal API and reserves the right to modify or delete it in the future. Use at your own risk.",
	    DiagnosticId = "XAOBS001")
	partial class DrawableWrapper : Drawable {
	}

The new `[Obsolete]` usage also specifies a custom warning code of
`XAOBS001` so that the warning can be suppressed if desired.

Add support for a new `$(AndroidEnableRestrictToAttributes)` MSBuild
"enum-style" property.  Supported values include:

  * `obsolete`: `generator --lang-features=restrict-to-attributes`
    will be used and the `[Obsolete]` custom attribute will be placed
    on bindings of Java types which have a `@RestrictTo` annotation.

    This is the default value.

  * `disable`: Java types with a `@RestrictTo` annotation
    will *not* be marked as `[Obsolete]`, and will be bound as a
    "normal" Java `public` type.

        <AndroidEnableRestrictToAttributes>disable</AndroidEnableRestrictToAttributes>

If you would instead prefer that types with `@RestrictTo` not be
bound at all, this can be achieved via Metadata, e.g.

	<remove-node path="//*[@annotated-visibility]" />

[0]: https://developer.android.com/reference/androidx/annotation/RestrictTo