[Android] Allow Previewer to set a per-page Context (#1304)

* Allow previewer to set a per-page context

* Remove unnecessary using directive

Xamarin.Forms.Platform.Android/Platform.cs

@@ -54,8 +54,6 @@ namespace Xamarin.Forms.Platform.Android
 
 		readonly bool _embedded;
 
-		
-
 		internal Platform(Context context, bool embedded)
 		{
 			_embedded = embedded;
@@ -305,7 +303,8 @@ namespace Xamarin.Forms.Platform.Android
 		[Obsolete("CreateRenderer(VisualElement) is obsolete as of version 2.5. Please use CreateRendererWithContext(VisualElement, Context) instead.")]
 		public static IVisualElementRenderer CreateRenderer(VisualElement element)
 		{
-			return CreateRenderer(element, Forms.Context);
+			// If there's a previewer context set, use that when created 
+			return CreateRenderer(element, GetPreviewerContext(element) ?? Forms.Context);
 		}
 
 		internal static IVisualElementRenderer CreateRenderer(VisualElement element, Context context)
@@ -1082,6 +1081,9 @@ namespace Xamarin.Forms.Platform.Android
 
 		#region Previewer Stuff
 		
+		internal static readonly BindableProperty PageContextProperty = 
+			BindableProperty.CreateAttached("PageContext", typeof(Context), typeof(Platform), null);
+
 		internal Platform(Context context) : this(context, false)
 		{
 			// we have this overload instead of using a default value for 
@@ -1090,7 +1092,21 @@ namespace Xamarin.Forms.Platform.Android
 
 		internal static void SetPageContext(BindableObject bindable, Context context)		
  		{
-			// We need to keep this around for now because the previewer calls it
+			// Set a context for this page and its child controls
+			bindable.SetValue(PageContextProperty, context);
+		}
+		
+		static Context GetPreviewerContext(Element element)
+		{
+			// Walk up the tree and find the Page this element is hosted in
+			Element parent = element;
+			while (!Application.IsApplicationOrNull(parent.RealParent))
+			{
+				parent = parent.RealParent;
+			}
+
+			// If a page is found, return the PageContext set by the previewer for that page (if any)
+			return (parent as Page)?.GetValue(PageContextProperty) as Context;
 		}
 
 		#endregion