Improve initial unlock delay (1/2)

When checking for the credentials, we add a new callback
onEarlyVerified which gets called as soon as we know that the
credential was correct.

In KeyguardUpdateMonitor, we track the unlocked state of the user,
and if it's still locked, we slow down all the transitions to allow
for a more gradual unlock experience.

Bug: 29007436

Change-Id: I406d228f9f3e41e07fe3292a61df175a7f579e4d