gd: Ensure main menu always show above the toolbar

This CL:
- ensures the main menu always stack above the toolbar when updating UI
  for Game Controls.
- adds unit test to check the z-order of the main menu and the toolbar.

Bug: b/329642687
Test: ash_unittests --gtest_filter="*GameDashboardContext*"
Change-Id: Iea6584d68509c683133d4f11887943ff9ac608e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5377973


Reviewed-by: Prameet Shah <phshah@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Cici Ruan <cuicuiruan@google.com>
Cr-Commit-Position: refs/heads/main@{#1277108}