[AndroidManifest] Add support for <property> element via PropertyAttribute.

[jpobst]

Fixes: #8729

Adds support for specifying the AndroidManifest.xml <property> element via a new [Property] attribute. This appears to be the exact same as <meta-data> but with a different name, so it is implemented exactly the same.

Example:

[Service (Name = "fooService", ForegroundServiceType = Android.Content.PM.ForegroundService.TypeSpecialUse)]
[Property ("android.app.PROPERTY_SPECIAL_USE_FGS_SUBTYPE", Value = "explanation_for_special_use")]
public class FooService : Service
{ ... }

creates:

<service android:name="fooService" android:foregroundServiceType="specialUse">
  <property android:name="android.app.PROPERTY_SPECIAL_USE_FGS_SUBTYPE"
      android:value="explanation_for_special_use" />
</service>

This element can be applied to <application>, <activity>, <provider>, <receiver>, and <service> elements.

[jonathanpeppers]

I didn't really have any concerns except the classes at the bottom of a file. 👍

[jonathanpeppers]

Should we put these in a separate file, to make them easier to find? I could see another test wanting to use these at some point, and they are useable within the entire project.

[jonathanpeppers]

This shouldn't be fixed here, but I wonder if the N Assemblies.SelectMany() calls could be a performance issue.

[jonpryor]

Proposed commit message:

Fixes: https://github.com/xamarin/xamarin-android/issues/8729

API-31 added support for a [`<property/>`][0] element within
`AndroidManifest.xml`, which can be contained within `<activity/>`, 
`<application/>`, `<provider/>`, `<receiver/>`, and `<service/>`
elements.

Add a new `Android.App.PropertyAttribute` custom attribute which will
emit `<property/>` within `AndroidManifest.xml`:

	[AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class,
	        AllowMultiple = true, Inherited = false)]
	public partial class PropertyAttribute : Attribute {

	    public string Name      {get;}
	    public string? Resource {get;}
	    public string? Value    {get;}

	    public PropertyAttribute (string name);
	}

Semantics otherwise appear to be identical to
`<meta-data/>`/`MetaDataAttribute`: only `Resource` or `Value` can
be specified, etc.  (This is checked by `aapt2`.)

Using `[assembly:Property(…)]` will result in `<property/>` being
emitted within the `<application/>` element.

For example:

	[Service (Name = "fooService", ForegroundServiceType = Android.Content.PM.ForegroundService.TypeSpecialUse)]
	[Property ("android.app.PROPERTY_SPECIAL_USE_FGS_SUBTYPE", Value = "explanation_for_special_use")]
	public partial class FooService : Service {
	}

will emit the `AndroidManifest.xml` fragment:

	<service android:name="fooService" android:foregroundServiceType="specialUse">
	  <property
	      android:name="android.app.PROPERTY_SPECIAL_USE_FGS_SUBTYPE"
	      android:value="explanation_for_special_use"
	  />
	</service>

[0]: https://developer.android.com/guide/topics/manifest/property-element

[entdark]

Hi, what version will it appear in?

[jpobst]

It will be in .NET 9 Preview 7.