[tests] Windows not writing AndroidManifest.xml to disk

When debugging this particular test on Windows, it was never writing
`AndroidManifest.xml` to disk after the first build.

This expression was false, because the times are equal:
https://github.com/xamarin/xamarin-android/blob/master/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/XamarinProject.cs#L352

Modifying the `AndroidManifest` property to clear the `Timestamp` on
the project item, forces the file to be rewritten, and the test now
passes on Windows.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/XamarinAndroidApplicationProject.cs

@@ -111,7 +111,19 @@ public AndroidLinkMode AndroidLinkModeRelease {
 			set { SetProperty (ReleaseProperties, KnownProperties.AndroidLinkMode, value.ToString ()); }
 		}
 
-		public string AndroidManifest { get; set; }
+		string androidManifest;
+
+		public string AndroidManifest {
+			get { return androidManifest; }
+			set {
+				androidManifest = value;
+
+				var item = OtherBuildItems?.FirstOrDefault (i => i.Include() == "Properties\\AndroidManifest.xml");
+				if (item != null) {
+					item.Timestamp = null;
+				}
+			}
+		}
 		public string LayoutMain { get; set; }
 		public string MainActivity { get; set; }
 		public string StringsXml { get; set; }