[fenix] Fixes https://github.com/mozilla-mobile/fenix/issues/5104: Changed empty_exception_container to match constraints.

This bug occurs because the activity/fragment is not recreated on
orientation change. A simple fix was to just set width and height to 0dp
to let it match constraints.

app/src/main/res/layout/fragment_site_permissions_exceptions.xml

@@ -35,8 +35,9 @@
 
     <TextView
         android:id="@+id/empty_exception_container"
-        android:layout_width="wrap_content"
-        android:layout_height="wrap_content"
+        android:layout_width="0dp"
+        android:layout_height="0dp"
+        android:gravity="center"
         android:text="@string/no_site_exceptions"
         android:textColor="?primaryText"
         android:textSize="20sp"