fix: Fix lazy theme dictionary caching

Author: Youssef1313

src/Uno.UI.Tests/Windows_UI_Xaml/Given_ResourceDictionary.cs

@@ -875,6 +875,26 @@ public void When_Default_Theme_Dictionary_Should_Be_Used_After_Removing_Active_T
 			Assert.IsFalse(dictionary.TryGetValue("TestKey", out _));
 		}
 
+		[TestMethod]
+		public void When_Lazy()
+		{
+			var dictionary = new ResourceDictionary();
+			var brush = new SolidColorBrush { Color = Colors.Red };
+			dictionary.TryGetValue("TestKey", out _);
+
+			dictionary.ThemeDictionaries.Add("Light", new WeakResourceInitializer(new Application(), o =>
+				new ResourceDictionary
+				{
+					["TestKey"] = new WeakResourceInitializer(o, _ => brush)
+				}));
+
+
+			// Make sure the cache is updated.
+			Assert.IsTrue(dictionary.TryGetValue("TestKey", out var testValue));
+			Assert.AreEqual(brush, testValue);
+
+		}
+
 		[TestMethod]
 		public void When_Resource_NotImplemented()
 		{

src/Uno.UI/UI/Xaml/ResourceDictionary.cs

@@ -249,6 +249,7 @@ private void Set(in ResourceKey resourceKey, object value, bool throwIfPresent)
 			{
 				_hasUnmaterializedItems = true;
 				_values[resourceKey] = new LazyInitializer(ResourceResolver.CurrentScope, resourceInitializer);
+				ResourceDictionaryValueChange?.Invoke(this, EventArgs.Empty);
 			}
 			else
 			{