Fix multiple onScreenTurnedOn callbacks in KeyguardHostView.

The show() method was calling onScreenTurnedOn() which attempted
to update the entire state.

Instead, show() should just ensure the current security screen
is showing.

This was causing some mischief in Face Unlock because onResume() was
being called multiple times.

Change-Id: I39682a213b1cc4dcf8133907b643028e844d0f47