flags: clean up keyboard lock flags

This change removes:
  keyboard-lock-api
and assigns:
  system-keyboard-lock
and removes backing features & tests for the former.

Bug: None
Change-Id: I4fe8fa3aeef65e2996e0091d557182af9fa6e565
Reviewed-on: https://chromium-review.googlesource.com/c/1477790
Reviewed-by: Joe Downing <joedow@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633332}
14 files changed