fix(onboarding-follow-suggestions): use the tabbar background drawable there

This makes the fragment look good

mastodon/src/main/res/layout/fragment_onboarding_follow_suggestions.xml

@@ -36,7 +36,7 @@
 		android:id="@+id/button_bar"
 		android:layout_width="match_parent"
 		android:layout_height="wrap_content"
-		android:background="?colorM3SurfaceVariant"
+		android:background="@drawable/bg_tabbar"
 		android:outlineProvider="bounds"
 		android:orientation="horizontal"
 		android:elevation="0dp">