Add preinstalled container web app to default shelf pins

For existing users, that they had default pin layout applied, add logic
that pins the container app in the first slot in the shelf when the app
first becomes available. When the default pin position for the app is
set, remember this in syncable prefs to avoid repinning the app after
user unpins it.

BUG=b/329710469

Change-Id: I1a3fd169ed398cb0e9e48577c9c5785685e0b309
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5408442


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Reviewed-by: Victor Vianna <victorvianna@google.com>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283249}