lacros: Register chrome app keepalive at right time.

The current implementation of chrome apps requires Lacros to always be
running. The current implementation generates the keep alive at the
wrong time [at the login screen] rather than after login. This CL fixes
the keep alive timing. In the process, this CL makes it so that
StandaloneBrowserExtensionApps is only generated if a client [e.g.
lacros] registers as a chrome app publisher. This means that another
entity is responsible for starting Lacros for the first time after
login.

Bug: 1278541
Change-Id: If2f9279351ef1f421f237cb61dd3a01dcd96babe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3489688
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#983369}