[tests] Update ManifestTest.cs

Context: https://github.com/xamarin/xamarin-android/blob/f1d59181c8daaa8d2abcdfd151b592ece49155ca/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/ManifestTest.cs#L650-L651

The `ModifyManifest` test actually writes "too much" to the
`AndroidManifest.xml` file, removing the `<application>` element,
causing the latest version of `bundletool` to error with:

    BT One element <application> was expected, but none were found.

To solve this, let's just set `application:versionCode` in the test
instead.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/ManifestTest.cs

@@ -665,14 +665,11 @@ public void ModifyManifest ([Values (true, false)] bool isRelease)
 	    <Namespace Prefix=""android"" Uri=""http://schemas.android.com/apk/res/android"" />
 	</Namespace>
 </PropertyGroup>
-<ItemGroup>
-	<_Permissions Include=""&lt;uses-permission android:name=&quot;android.permission.READ_CONTACTS&quot; /&gt;"" />
-</ItemGroup>
 <Target Name=""_Foo"">
 	<XmlPoke
 		XmlInputPath=""$(IntermediateOutputPath)android\AndroidManifest.xml""
-		Value=""@(_Permissions)""
-		Query=""/manifest""
+		Value=""12345""
+		Query=""/manifest/@android:versionCode""
 		Namespaces=""$(Namespace)""
 	/>
 </Target>
@@ -684,7 +681,7 @@ public void ModifyManifest ([Values (true, false)] bool isRelease)
 			using (var builder = CreateApkBuilder (Path.Combine ("temp", TestName))) {
 				Assert.IsTrue (builder.Build (proj), "Build should have succeeded");
 				var manifest = builder.Output.GetIntermediaryAsText (Root, Path.Combine ("android", "AndroidManifest.xml"));
-				Assert.IsTrue (manifest.Contains ("READ_CONTACTS"), $"Manifest should contain the READ_CONTACTS");
+				Assert.IsTrue (manifest.Contains ("12345"), $"Manifest should contain versionCode=12345");
 			}
 		}