Support slide to udfps auth on keyguard

- Allow udfps fingerprint to run when the bouncer is about to show;
instead, we'll manually pause auth in UdfpsKeyguardViewController when
we don't want to allow the user to authenticate
- On Keyguard, when bringing up the bouncer (swipe up on LS), continue
to show the udfps target until it hits a threshold - then we fade out
the icon

Test: manual
Bug: 178509275
Change-Id: I6e883a619b38a1a136f9d6e39b755a75c5028553