Revert "Expose `FaceManager` APIs as `@SystemApi`."

Revert submission 25750009-facemanager_20240110

Reason for revert: Project cancelled.

Reverted changes: /q/submissionid:25750009-facemanager_20240110

Bug: 329561487

Test: Local
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:5fe554bfd9f28b7b15677dfb74c756f827074ebc)
Merged-In: I7e329f311b3eb599195f415cf39dc28bb11139a1
Change-Id: I7e329f311b3eb599195f415cf39dc28bb11139a1
Revert FaceManager changes.