[Core] Unify LinearGradientBrush default values with WPF (#11575)

* Unify LinearGradientBrush default values with WPF

* Updated unit test

fixes #11569

Xamarin.Forms.Core.UnitTests/LinearGradientBrushTests.cs

@@ -18,7 +18,7 @@ namespace Xamarin.Forms.Core.UnitTests
 			LinearGradientBrush linearGradientBrush = new LinearGradientBrush();
 
 			Assert.AreEqual(1.0d, linearGradientBrush.EndPoint.X, "EndPoint.X");
-			Assert.AreEqual(0.0d, linearGradientBrush.EndPoint.Y, "EndPoint.Y");
+			Assert.AreEqual(1.0d, linearGradientBrush.EndPoint.Y, "EndPoint.Y");
 		}
 
 		[Test]

Xamarin.Forms.Core/LinearGradientBrush.cs

@@ -38,7 +38,7 @@
 		}
 
 		public static readonly BindableProperty EndPointProperty = BindableProperty.Create(
-			nameof(EndPoint), typeof(Point), typeof(LinearGradientBrush), new Point(1, 0));
+			nameof(EndPoint), typeof(Point), typeof(LinearGradientBrush), new Point(1, 1));
 
 		public Point EndPoint
 		{