[android] cache `Join` and `Cap` enum values (#24248)

Context: https://github.com/dotnet/maui/issues/23991
Context: https://github.com/chabiss/periodictable

In the above sample, a lot of time is spent in:

    921.46ms (4.3%) mono!Android.Graphics.Paint.Cap.get_Butt()
    872.40ms (4.1%) mono!Android.Graphics.Paint.Join.get_Miter()

This exposes a performance issue with `Java.Lang.Enum` values:

* `java.lang.Enum` in Java are objects (not `int` like C#)

* When accessing an enum value, Java returns an object we have to wrap
  in a C# object.

* .NET for Android has to do bookkeeping around this, lookup in a hash
  table, etc.

To avoid this, we can store the `Join` and `Cap` values in a static
field and avoid calling into Java. This approach is already working
in .NET MAUI for `ImageView.ScaleType`:

9361f90a5d/src/Core/src/Platform/Android/AspectExtensions.cs (L7-L10)

After this change, the time spent is completely gone:

    2.41ms (0.02%) mono.android!Android.Graphics.Paint.Join.get_Miter()

I can't find the same call for (the unfortunately named) `get_Butt()`
at all.

In the future, we might consider changing the C# binding for
`Java.Lang.Enum` to "auto-cache" values in C# static fields. Not sure
if there is enough time left for it to happen in .NET 9, though.

src/Core/src/Graphics/MauiDrawable.Android.cs

@@ -15,6 +15,14 @@ namespace Microsoft.Maui.Graphics
 {
 	public class MauiDrawable : PaintDrawable
 	{
+		static Join? JoinMiter;
+		static Join? JoinBevel;
+		static Join? JoinRound;
+
+		static Cap? CapButt;
+		static Cap? CapSquare;
+		static Cap? CapRound;
+
 		readonly AContext? _context;
 		readonly float _density;
 
@@ -298,20 +306,13 @@ namespace Microsoft.Maui.Graphics
 
 		public void SetBorderLineJoin(LineJoin lineJoin)
 		{
-			Join? aLineJoin = Join.Miter;
-
-			switch (lineJoin)
+			Join? aLineJoin = lineJoin switch
 			{
-				case LineJoin.Miter:
-					aLineJoin = Join.Miter;
-					break;
-				case LineJoin.Bevel:
-					aLineJoin = Join.Bevel;
-					break;
-				case LineJoin.Round:
-					aLineJoin = Join.Round;
-					break;
-			}
+				LineJoin.Miter => JoinMiter ??= Join.Miter,
+				LineJoin.Bevel => JoinBevel ??= Join.Bevel,
+				LineJoin.Round => JoinRound ??= Join.Round,
+				_ => JoinMiter ??= Join.Miter,
+			};
 
 			if (_strokeLineJoin == aLineJoin)
 				return;
@@ -323,20 +324,13 @@ namespace Microsoft.Maui.Graphics
 
 		public void SetBorderLineCap(LineCap lineCap)
 		{
-			Cap? aLineCap = Cap.Butt;
-
-			switch (lineCap)
+			Cap? aLineCap = lineCap switch
 			{
-				case LineCap.Butt:
-					aLineCap = Cap.Butt;
-					break;
-				case LineCap.Square:
-					aLineCap = Cap.Square;
-					break;
-				case LineCap.Round:
-					aLineCap = Cap.Round;
-					break;
-			}
+				LineCap.Butt => CapButt ??= Cap.Butt,
+				LineCap.Square => CapSquare ??= Cap.Square,
+				LineCap.Round => CapRound ??= Cap.Round,
+				_ => CapButt ??= Cap.Butt,
+			};
 
 			if (_strokeLineCap == aLineCap)
 				return;