Updater: Install the new `Keystone` bundle with the Updater.

This change creates a new app bundle named "GoogleSoftwareUpdate".
The bundle contains the replacement for the "ksadmin" tool, used by
Keystone's registration framework. The structure is as follows:
GoogleUpdater
  Contents
    Helpers
      ksadmin
    Info.plist
    MacOS
      GoogleSoftwareUpdate
    PkgInfo

Note: As there are many references to GoogleSoftwareUpdate.bundle and
ultimately we are trying to provide an in-place replacement for it,
I had to change the "mac_app_bundle" template to support the production
of a ".bundle".

Bug: 1247725
Change-Id: Ib717aab5682a04d84e50334f5b63f458c98c8a33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3149833


Commit-Queue: Mila Green <milagreen@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/main@{#920275}