Keyguard: Clean up security message display

- Remove timinig out of messages. This is no longer
  necessary because we do not show the owner and charging
  info in that area anymore.
- Remove the "important" parameter, which was equivalent to
  clearing the message if false.

Change-Id: I5fae5432724ad90a68f53f678f933c4a15dc6e9b
Fixes: 28270749
Test: manual; reboot device, swipe up, verify message doesn't disappear after 5s