Fix Selection Indicator for GridView sample (microsoft#1385)

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## Description
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Due to a missing BasedOn, the selection indicator and other interface
characteristics where missing on the custom layout sample.
## Motivation and Context
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<!--- If it fixes an open issue, please link to the issue here. -->
Fixes microsoft#1072
## How Has This Been Tested?
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## Screenshots (if appropriate):

## Types of changes
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the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

WinUIGallery/ControlPages/GridViewPage.xaml

@@ -104,7 +104,7 @@
                 <GridView x:Name="StyledGrid"
                           ItemTemplate="{StaticResource ImageOverlayTemplate}">
                     <GridView.ItemContainerStyle>
-                        <Style TargetType="GridViewItem">
+                        <Style TargetType="GridViewItem" BasedOn="{StaticResource DefaultGridViewItemStyle}">
                             <Setter Property="Margin" Value="5"/>
                         </Style>
                     </GridView.ItemContainerStyle>
@@ -155,7 +155,7 @@ some parts of the GridViewItems (i.e. the margins). --&gt;
     ItemTemplate="{StaticResource ImageOverlayTemplate}"&gt;
 
     &lt;GridView.ItemContainerStyle&gt;
-        &lt;Style TargetType="GridViewItem"&gt;
+        &lt;Style TargetType="GridViewItem" BasedOn="{StaticResource DefaultGridViewItemStyle}"&gt;
             &lt;Setter Property="Margin" Value="$(ColMargin), $(RowMargin), $(ColMargin), $(RowMargin)"/&gt;
         &lt;/Style&gt;
     &lt;/GridView.ItemContainerStyle&gt;