Support Flex Auto Enrollment in legacy state determination flow

AutoEnrollmentTypeChecker first verifies that the flex enrollment token
is present, halting state determination on Flex if the token isn't
found. AutoEnrollmentClientImpl will then try to obtain the flex token
during initialization for initial enrollment. If present, it will
proceed through a modified state determination flow, skipping the PSM
check and proceeding directly to a state retrieval request, including
the flex token.

Note that after some discussion in crrev/c/5311158, we're planning not
to skip the PSM check for unified state determination. In the legacy
flow however, the FRE mechanism is different and separate, so Flex
devices should never need to make a PSM check here.

Bug: b/320497330
Test: TODO: Update with unittests command
Change-Id: I717ca8d56865871236d388dc5cfd248eed49e50b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5356400


Reviewed-by: Artem Sumaneev <asumaneev@google.com>
Commit-Queue: Jackson Tadie <jacksontadie@google.com>
Reviewed-by: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280470}