Don't check the keyguard PIN a second time

If you tap the enter key twice on the keyguard PIN entry
screen, the second tap can sometimes register because keyguard
hasn't been dismissed yet.  The fix is to ignore a second attempt
after the PIN has already been verified.

Fixes bug 19216025

Change-Id: I73a004e91e43f3da65ec84f90de6943e717d737f