LockSettingsService: clear gatekeeper state when removing user

On some devices gatekeeper would fail after certain enrollments due to
some persistent state left around when removing users.

Bug: 224397241
Fixes: 224397241
Test: atest com.android.server.locksettings
Change-Id: I49861e11b265b56c395cdb25eef13c8b3a33ef19