sunfish: Fix search box button visibility when enabled pref is changed

https://crrev.com/c/5982018 introduced logic to hide the search box
entry point when the enabled pref is false. However, this mistakenly did
not add an observer on the pref to `AppListControllerImpl`, so when the
pref is changed, the search box entry point is still shown until
`UpdateSearchBoxUiVisibilities` is next called - often on next log in.

Add an observer on the Sunfish pref in `AppListControllerImpl` to fix
this issue.

Bug: b:368184169
Change-Id: I6a6a210077093bb08f586eb055127110036b5cd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5997733
Commit-Queue: Michael Cui <mlcui@google.com>
Reviewed-by: Toni Barzic <tbarzic@google.com>
Cr-Commit-Position: refs/heads/main@{#1380701}