GitLab

This CL pulls the System Web App migration logic into PendingAppInstallTask
in preparation for External App migration. This also changes migration_source
to a list of apps that the Web App System will uninstall using the App Service
and then migrate OS data from.

This CL also adds the Settings SWA as a migration target for OS Settings.

Bug: 976578
Change-Id: I1d938690d7550101138563a3b266bc491fefd2df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1736129
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688403}