[One .NET] Do not always preserve Android.Runtime.Extensions (#5365)

Context: #5167

The `Android.Runtime.Extensions` type is not accessed via reflection.

It *is* preserved by the linker when needed, e.g.:

	$ illinkanalyzer -r Android.Runtime.Extensions linker-dependencies.xml.gz
	…
	--- Method:TResult Android.Runtime.Extensions::JavaCast(Android.Runtime.IJavaObject) dependencies ---
	Dependency #1
	        Method:TResult Android.Runtime.Extensions::JavaCast(Android.Runtime.IJavaObject)
	        | MethodSpec:TResult Android.Runtime.Extensions::JavaCast<T>(Android.Runtime.IJavaObject) [1 deps]
	        | Method:T Android.App.Activity::FindViewById(System.Int32) [1 deps]
	        | MethodSpec:!!0 Android.App.Activity::FindViewById<Android.Widget.Button>(System.Int32) [1 deps]
	        | Method:System.Void UnnamedProject.MainActivity::OnCreate(Android.OS.Bundle) [2 deps]
	        | Assembly:UnnamedProject, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null [1 deps]
	        | Other:Copy
	…

src/Microsoft.Android.Sdk.ILLink/PreserveLists/Mono.Android.xml

@@ -5,7 +5,6 @@
 		<type fullname="Android.Runtime.AnnotationAttribute" />
 		<type fullname="Android.Runtime.CharSequence" />
 		<type fullname="Android.Runtime.ConstructorBuilder" />
-		<type fullname="Android.Runtime.Extensions" />
 		<type fullname="Android.Runtime.GeneratedDummyHost" />
 		<type fullname="Android.Runtime.GeneratedEnumAttribute" />
 		<type fullname="Android.Runtime.IJavaObject" />