[fenix] Fixes https://github.com/mozilla-mobile/fenix/issues/8491: Changed toolbar background for PBM when is set to top

This change was required so that the background of the toolbar match the
homeFragment gradient

app/src/main/res/drawable/private_home_bottom_bar_background_gradient_top.xml

@@ -6,12 +6,7 @@
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
 <item>
     <shape>
-        <gradient
-            android:angle="45"
-            android:startColor="#7529A7"
-            android:centerColor="#492E85"
-            android:endColor="#383372"
-            android:type="linear" />
+        <solid android:color="@color/toolbar_end_gradient_private_theme"/>
     </shape>
 </item>
 <item android:top="-2dp" android:left="-2dp" android:right="-2dp">