Fix the empty preference after clicking gear icon

Because there is no other options for 'None' or 'Swipe, there is no
necessary for showing the gear icon to show options.

'Lock screen message' could be found in
Settings > Display > Lock screen display.

Add testcase to verify the ChangeScreenLockPreferenceController's
behavior.

Test: make ROBOTEST_FILTER=ChangeScreenLockPreferenceControllerTest \
      RunSettingsRoboTests -j40
Change-Id: Icdcd672261749d106162053d6f5228cee420a810
Fixes: 110848852