Updater: Don't delete keystone shim launch plists on updates.

Only delete them if the updater is entirely uninstalled.

Also, brand them, to avoid Edge's (or others') implementation from
altering the install.

Also, clean up a double-deletion of the install directory.

Bug: 1440706
Change-Id: I432df151a15e0af84ee65d98ef9d48891bb25900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4519160
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Xiaoling Bao <xiaolingbao@chromium.org>
Reviewed-by: Xiaoling Bao <xiaolingbao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1141679}