Fix incorrect background property name on Win 8.1 phone and tablet styles (#329)

Xamarin.Forms.Platform.WinRT.Phone/PhoneResources.xaml

@@ -436,8 +436,7 @@
 							<RowDefinition Height="*" />
 						</Grid.RowDefinitions>
 						
-                        <Grid Height="79" Background="{TemplateBinding NavigationBarBackground}" 
-							  Visibility="{TemplateBinding TitleVisibility}">
+						<Grid Height="79" Background="{TemplateBinding ToolbarBackground}"  Visibility="{TemplateBinding TitleVisibility}">
                             <TextBlock Margin="10,0,0,0" Name="title" Foreground="{TemplateBinding TitleBrush}" VerticalAlignment="Center" Style="{ThemeResource HeaderTextBlockStyle}" Text="{Binding Title}" />
                         </Grid>
                    		<ContentPresenter x:Name="presenter" Grid.Row="1" ContentTransitions="{TemplateBinding ContentTransitions}" />

Xamarin.Forms.Platform.WinRT.Tablet/TabletResources.xaml

@@ -327,8 +327,7 @@
 							<RowDefinition Height="*" />
 						</Grid.RowDefinitions>
 
-						<Grid Grid.Row="0" Grid.Column="0" Height="79" VerticalAlignment="Center" Background="{TemplateBinding NavigationBarBackground}" 
-							  Visibility="{TemplateBinding TitleVisibility}">
+						<Grid Grid.Row="0" Grid.Column="0" Height="79" VerticalAlignment="Center" Background="{TemplateBinding ToolbarBackground}" Visibility="{TemplateBinding TitleVisibility}">
 							<Grid.ColumnDefinitions>
 								<ColumnDefinition Width="Auto" MinWidth="{Binding TitleInset,RelativeSource={RelativeSource TemplatedParent}}" />
 								<ColumnDefinition Width="*" />