[MSan] Fix use-after-dtor errors in ash::Shell

ash::Shell transitively owns many objects, some of which may observe
ash::Shell itself. Reorder `shell_observers_` to be the first field, so
that transitively-owned objects that attempt to unregister themselves as
a shell observer at destruction time don't use the ObserverList after it
is already destroyed.

Bug: 40222690
Change-Id: Ic10dacdb8b3ec8f489fe0ed8c7f1484fe9f0e6de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5672229
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1322426}