[iOS] Display overflow action customization in one list

The new design has all actions together in one section and no
auto-reordering when an item is toggled between shown and hidden. This
CL makes most of its changes in the customization model, to remove
the auto-moving, but also makes a few UI changes to only display one
section of actions.

Fixed: 1476063, b/297397110
Change-Id: If63d5ccf9923a4a5e245e7b23879ba9090ed4f93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4811726
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Cr-Commit-Position: refs/heads/main@{#1189146}