Credential FRP: remove GateKeeper

From O-DR, all new credentials will be synthetic password based. A
GateKeeper credential could be enrolled but that would require
`adb shell cmd lock_settings sp 0`.

Credential FRP won't be released before O-DR so there is no need to
handle GateKeeper credentials. The protocol constants have been updated
because they are not yet in use.

Bug: 36814845
Test: runtest frameworks-services -p com.android.server.locksettings
Change-Id: Id0def06bb56fef47f3151f4f5cd0db738b35979f