Visual element packager (#2775)

* [iOS] Enable ShouldRecognizeSimultaneously on PanGesture

fixes #2299

* Add test case for 2296

* Fix test case name/number

* Convert fix to opt-in platform specific

* Update test case to use platform specific

* Fixes insane crash that sometimes happens when adding views.

* Added additional null checks for renderers

* Adde check for disposed object

* [Core] Fix ChildAdd Linq on VEP

Xamarin.Forms.Platform.Android/Cells/CellRenderer.cs

@@ -95,6 +95,8 @@ namespace Xamarin.Forms.Platform.Android
 
 			_onForceUpdateSizeRequested = (sender, e) => 
 			{
+				if (nativeCell.Handle == IntPtr.Zero)
+					return;
 				// RenderHeight may not be changed, but that's okay, since we
 				// don't actually use the height argument in the OnMeasure override.
 				nativeCell.Measure(nativeCell.Width, (int)cell.RenderHeight);

Xamarin.Forms.Platform.Android/VisualElementPackager.cs

@@ -5,6 +5,7 @@ using Android.Content;
 using Xamarin.Forms.Internals;
 using Android.Views;
 using AView = Android.Views.View;
+using System.Linq;
 
 namespace Xamarin.Forms.Platform.Android
 {
@@ -109,7 +110,7 @@ namespace Xamarin.Forms.Platform.Android
 				IVisualElementRenderer renderer = oldRenderer;
 				if (pool != null)
 					renderer = pool.GetFreeRenderer(view);
-				if (renderer == null)
+				if (renderer == null || (renderer.View?.Handle ?? IntPtr.Zero) == IntPtr.Zero)
 				{
 					Performance.Start(reference, "New renderer");
 					renderer = Platform.CreateRenderer(view, _renderer.View.Context);
@@ -118,7 +119,7 @@ namespace Xamarin.Forms.Platform.Android
 
 				if (renderer == oldRenderer)
 				{
-					Platform.SetRenderer(renderer.Element, null);
+					renderer.Element?.ClearValue(Platform.RendererProperty);
 					renderer.SetElement(view);
 				}
 
@@ -158,7 +159,7 @@ namespace Xamarin.Forms.Platform.Android
 			if (view != null)
 				AddChild(view);
 
-			if (ElementController.LogicalChildren[ElementController.LogicalChildren.Count - 1] != view)
+			if (ElementController.LogicalChildren.LastOrDefault() != view)
 				EnsureChildOrder();
 		}