Skip enrollment complete screen during Chromad migration

During the Chromad to cloud management migration, after the powerwash,
the device is expected to auto re-enroll. After that, the device should
skip the "Enterprise enrollment complete" screen. This requirement is
indicated by a flag file, written by the session_manager before starting
the powerwash.

The enrollment screen checks (async) whether this flag file exists or
not, to decide if the final step (enrollment complete screen) should be
skipped.

Bug: 1209246
Change-Id: Iac526a1c015cef9a6b63e070acc754fa0b7b817b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3452270


Reviewed-by: Roman Sorokin <rsorokin@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Felipe Andrade <fsandrade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#970711}