Set the DropDownGlyphPath Fill color directly in checked states

This is required for Avalonia after the switch to Path instead of TextBlock as in WinUI. WinUI glyph TextBlock just inherits the foreground color of the secondary button so doesn't need this.

src/Avalonia.Themes.Fluent/Controls/SplitButton.xaml

@@ -250,6 +250,9 @@
   <Style Selector="SplitButton:checked /template/ Button#PART_SecondaryButton">
     <Setter Property="Foreground" Value="{DynamicResource SplitButtonForegroundChecked}" />
   </Style>
+  <Style Selector="SplitButton:checked /template/ Path#DropDownGlyphPath">
+    <Setter Property="Fill" Value="{DynamicResource SplitButtonForegroundChecked}" />
+  </Style>
 
   <!-- CheckedFlyoutOpen State -->
   <Style Selector="SplitButton:checked-flyout-open /template/ Rectangle#PrimaryBackgroundRectangle">
@@ -267,6 +270,9 @@
   <Style Selector="SplitButton:checked-flyout-open /template/ Button#PART_SecondaryButton">
     <Setter Property="Foreground" Value="{DynamicResource SplitButtonForegroundCheckedPressed}" />
   </Style>
+  <Style Selector="SplitButton:checked-flyout-open /template/ Path#DropDownGlyphPath">
+    <Setter Property="Fill" Value="{DynamicResource SplitButtonForegroundCheckedPressed}" />
+  </Style>
 
   <!-- CheckedTouchPressed State -->
   <Style Selector="SplitButton:checked-touch-pressed /template/ Rectangle#PrimaryBackgroundRectangle">
@@ -284,6 +290,9 @@
   <Style Selector="SplitButton:checked-touch-pressed /template/ Button#PART_SecondaryButton">
     <Setter Property="Foreground" Value="{DynamicResource SplitButtonForegroundCheckedPressed}" />
   </Style>
+  <Style Selector="SplitButton:checked-touch-pressed /template/ Path#DropDownGlyphPath">
+    <Setter Property="Fill" Value="{DynamicResource SplitButtonForegroundCheckedPressed}" />
+  </Style>
 
   <!-- CheckedPrimaryPointerOver State -->
   <Style Selector="SplitButton:checked-primary-pointerover /template/ Border#Border">
@@ -302,6 +311,9 @@
   <Style Selector="SplitButton:checked-primary-pointerover /template/ Button#PART_SecondaryButton">
     <Setter Property="Foreground" Value="{DynamicResource SplitButtonForegroundChecked}" />
   </Style>
+  <Style Selector="SplitButton:checked-primary-pointerover /template/ Path#DropDownGlyphPath">
+    <Setter Property="Fill" Value="{DynamicResource SplitButtonForegroundChecked}" />
+  </Style>
 
   <!-- CheckedPrimaryPressed State -->
   <Style Selector="SplitButton:checked-primary-pressed /template/ Border#Border">
@@ -320,6 +332,9 @@
   <Style Selector="SplitButton:checked-primary-pressed /template/ Button#PART_SecondaryButton">
     <Setter Property="Foreground" Value="{DynamicResource SplitButtonForegroundChecked}" />
   </Style>
+  <Style Selector="SplitButton:checked-primary-pressed /template/ Path#DropDownGlyphPath">
+    <Setter Property="Fill" Value="{DynamicResource SplitButtonForegroundChecked}" />
+  </Style>
 
   <!-- CheckedSecondaryPointerOver State -->
   <Style Selector="SplitButton:checked-secondary-pointerover /template/ Border#Border">
@@ -338,6 +353,9 @@
     <Setter Property="BorderBrush" Value="{DynamicResource SplitButtonBorderBrushCheckedPointerOver}" />
     <Setter Property="Foreground" Value="{DynamicResource SplitButtonForegroundCheckedPointerOver}" />
   </Style>
+  <Style Selector="SplitButton:checked-secondary-pointerover /template/ Path#DropDownGlyphPath">
+    <Setter Property="Fill" Value="{DynamicResource SplitButtonForegroundCheckedPointerOver}" />
+  </Style>
 
   <!-- CheckedSecondaryPressed State -->
   <Style Selector="SplitButton:checked-secondary-pressed /template/ Border#Border">
@@ -356,6 +374,9 @@
     <Setter Property="BorderBrush" Value="{DynamicResource SplitButtonBorderBrushCheckedPressed}" />
     <Setter Property="Foreground" Value="{DynamicResource SplitButtonForegroundCheckedPressed}" />
   </Style>
+  <Style Selector="SplitButton:checked-secondary-pressed /template/ Path#DropDownGlyphPath">
+    <Setter Property="Fill" Value="{DynamicResource SplitButtonForegroundCheckedPressed}" />
+  </Style>
 
   <!--VisualStateGroup x:Name="SecondaryButtonPlacementStates">
     <VisualState x:Name="SecondaryButtonRight" />