Add PSM_SKIPPED_FOR_FAE PsmExecutionResult enum value

This value isn't planned to be used for any particular functionality
(although it won't hurt to have this scenario recorded in metrics), we
just add it to be exhaustive and enumerate the new scenario that Flex
Auto Enrollment introduces.

Bug: b/320497330
Test: CQ
Change-Id: If6a2bedc65d7135491b2900fda7930731af9a05d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5374536
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Reviewed-by: Artem Sumaneev <asumaneev@google.com>
Commit-Queue: Jackson Tadie <jacksontadie@google.com>
Cr-Commit-Position: refs/heads/main@{#1280052}