[android] avoid View.Context during ContextView scrolling (#8243)

Context: #8012
Context: https://github.com/Kalyxt/Test_CollectionView

We had some reports of poor `CollectionView` performance while
scrolling on an older Android device.

Reviewing `dotnet trace` output, I did find some issues similar to #8001:

    317.42ms (1.1%) mono.android!Android.Views.View.get_Context()

1% of the time is spent in repeated calls to `View.Context` inside the
`ItemContentView` class. Making a new overload for
`ContextExtensions.FromPixel()`, I was able to remove all of these
calls.

This results in only a couple `View.Context` calls on startup now,
much better:

    1.30ms (0.01%) mono.android!Android.Views.View.get_Context()

Using the "janky frames" metric from the latest profiler in Android
Studio Dolphin:

https://developer.android.com/studio/profile/jank-detection

With my slowest Android 12+ device, a Pixel 4a, I could actually see a
few "janky frames" while scrolling the sample.

With these changes in place, I only see 1 "janky frame" now.

I also compared the before and after with the visual GPU profiler:

https://developer.android.com/topic/performance/rendering/inspect-gpu-rendering

It appears at a glance that these changes are better.

I am unsure at what point the performance will be good enough to close
 #8012, but this helps!

Author: jonathanpeppers

src/Controls/src/Core/Handlers/Items/Android/ItemContentView.cs

@@ -65,7 +65,7 @@ protected override void OnLayout(bool changed, int l, int t, int r, int b)
 				return;
 			}
 
-			var size = Context.FromPixels(r - l, b - t);
+			var size = this.FromPixels(r - l, b - t);
 
 			//TODO: RUI Is this the best way?
 			//View.Arrange(new Rectangle(Point.Zero, size));
@@ -99,23 +99,23 @@ protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
 
 			var width = MeasureSpec.GetMode(widthMeasureSpec) == MeasureSpecMode.Unspecified
 				? double.PositiveInfinity
-				: Context.FromPixels(pixelWidth);
+				: this.FromPixels(pixelWidth);
 
 			var height = MeasureSpec.GetMode(heightMeasureSpec) == MeasureSpecMode.Unspecified
 				? double.PositiveInfinity
-				: Context.FromPixels(pixelHeight);
+				: this.FromPixels(pixelHeight);
 
 
 			var measure = View.Measure(width, height);
 
 			if (pixelWidth == 0)
 			{
-				pixelWidth = (int)Context.ToPixels(measure.Width);
+				pixelWidth = (int)this.ToPixels(measure.Width);
 			}
 
 			if (pixelHeight == 0)
 			{
-				pixelHeight = (int)Context.ToPixels(measure.Height);
+				pixelHeight = (int)this.ToPixels(measure.Height);
 			}
 
 			_reportMeasure?.Invoke(new Size(pixelWidth, pixelHeight));
@@ -144,10 +144,10 @@ void UpdateContentLayout()
 			if (mauiControlsView == null || aview == null)
 				return;
 
-			var x = (int)Context.ToPixels(mauiControlsView.X);
-			var y = (int)Context.ToPixels(mauiControlsView.Y);
-			var width = Math.Max(0, (int)Context.ToPixels(mauiControlsView.Width));
-			var height = Math.Max(0, (int)Context.ToPixels(mauiControlsView.Height));
+			var x = (int)this.ToPixels(mauiControlsView.X);
+			var y = (int)this.ToPixels(mauiControlsView.Y);
+			var width = Math.Max(0, (int)this.ToPixels(mauiControlsView.Width));
+			var height = Math.Max(0, (int)this.ToPixels(mauiControlsView.Height));
 
 			aview.Layout(x, y, width, height);
 

src/Controls/src/Core/Handlers/Items/Android/RecyclerViewScrollListener.cs

@@ -41,14 +41,12 @@ public override void OnScrolled(RecyclerView recyclerView, int dx, int dy)
 			_verticalOffset += dy;
 
 			var (First, Center, Last) = GetVisibleItemsIndex(recyclerView);
-
-			var context = recyclerView.Context;
 			var itemsViewScrolledEventArgs = new ItemsViewScrolledEventArgs
 			{
-				HorizontalDelta = context.FromPixels(dx),
-				VerticalDelta = context.FromPixels(dy),
-				HorizontalOffset = context.FromPixels(_horizontalOffset),
-				VerticalOffset = context.FromPixels(_verticalOffset),
+				HorizontalDelta = recyclerView.FromPixels(dx),
+				VerticalDelta = recyclerView.FromPixels(dy),
+				HorizontalOffset = recyclerView.FromPixels(_horizontalOffset),
+				VerticalOffset = recyclerView.FromPixels(_verticalOffset),
 				FirstVisibleItemIndex = First,
 				CenterItemIndex = Center,
 				LastVisibleItemIndex = Last

src/Core/src/Platform/Android/ContextExtensions.cs

@@ -32,13 +32,25 @@ public static class ContextExtensions
 		// TODO FromPixels/ToPixels is both not terribly descriptive and also possibly sort of inaccurate?
 		// These need better names. It's really To/From Device-Independent, but that doesn't exactly roll off the tongue.
 
+		internal static double FromPixels(this View view, double pixels)
+		{
+			if (s_displayDensity != float.MinValue)
+				return pixels / s_displayDensity;
+			return view.Context.FromPixels(pixels);
+		}
+
 		public static double FromPixels(this Context? self, double pixels)
 		{
 			EnsureMetrics(self);
 
 			return pixels / s_displayDensity;
 		}
 
+		internal static Size FromPixels(this View view, double width, double height)
+		{
+			return new Size(view.FromPixels(width), view.FromPixels(height));
+		}
+
 		public static Size FromPixels(this Context context, double width, double height)
 		{
 			return new Size(context.FromPixels(width), context.FromPixels(height));