Keyguard should authenticate with FP even after getting valid FP

There may be strange states where the user is already authenticated
but still on the lockscreen. The user should be able to dismiss
keyguard in that state.

Fixes: 29306222

Test: unlock phone
go back to keyguard
swipe up screen half way
touch fingerprint sensor (icon should change to unlocked)
then touch fingerprint sensor again (keyguard should be dismissed)

Test: unlock phone and go to settings -> Pixel Imprint
lock phone, unlock phone with fingerprint
touch sensor again and make sure Pixel Imprint page also responds to FP
do this test at least 10 times

Change-Id: I86acd520a06a68fab3548dd4cf6153a7833114fe