[Android] Allow to use system fonts (#15010)

src/Core/src/Fonts/FontManager.Android.cs

@@ -2,6 +2,7 @@
 using System.Collections.Concurrent;
 using System.IO;
 using Android.Graphics;
+using Android.Graphics.Fonts;
 using Android.Util;
 using Microsoft.Extensions.Logging;
 using AApplication = Android.App.Application;
@@ -130,6 +131,22 @@ public FontSize GetFontSize(Font font, float defaultFontSize = 0)
 			return null;
 		}
 
+		Typeface? LoadDefaultTypeface(string fontfamily)
+		{
+			switch (fontfamily.ToLowerInvariant())
+			{
+				case "monospace":
+					return Typeface.Monospace;
+				case "sansserif":
+				case "sans-serif":
+					return Typeface.SansSerif;
+				case "serif":
+					return Typeface.Serif;
+				default:
+					return null;
+			}
+		}
+
 		Typeface? CreateTypeface((string? fontFamilyName, FontWeight weight, bool italic) fontData)
 		{
 			var (fontFamily, weight, italic) = fontData;
@@ -140,7 +157,9 @@ public FontSize GetFontSize(Font font, float defaultFontSize = 0)
 
 			if (!string.IsNullOrWhiteSpace(fontFamily))
 			{
-				if (GetFromAssets(fontFamily) is Typeface typeface)
+				if (LoadDefaultTypeface(fontFamily) is Typeface systemTypeface)
+					result = systemTypeface;
+				else if (GetFromAssets(fontFamily) is Typeface typeface)
 					result = typeface;
 				else
 					result = Typeface.Create(fontFamily, style);

src/Core/tests/DeviceTests/Services/FontManagerTests.Android.cs

@@ -79,4 +79,20 @@ public void CanLoadEmbeddedFont()
 
 		Assert.False(expected.Equals(actual));
 	}
+
+	[Theory]
+	[InlineData("monospace")]
+	[InlineData("sansserif")]
+	[InlineData("serif")]
+	public void CanLoadSystemFonts(string fontName)
+	{
+		if (!OperatingSystem.IsAndroidVersionAtLeast(28))
+			return;
+
+		var registrar = new FontRegistrar(fontLoader: null);
+		var manager = new FontManager(registrar);
+		var actual = manager.GetTypeface(Font.OfSize(fontName, 12));
+
+		Assert.NotEqual(Typeface.Default, actual);
+	}
 }