Represent the Gallery screen hierarchy as a tree

This CL changes the Gallery app screens to be a tree instead of a list.
This will allow adding sub-menus to the gallery app in case we want
multiple different screens for a single feature, which is the case for
the People activity/screens.

Bug: 238993727
Test: Builds
Change-Id: Idd28fd5a0fa79fd32ab1f8a95ed8c16fa2f465ac