Let keyguard UI procced as soon as user password is validated

Notify the callback as soon as first stage password verification passes,
leaving the decryption of synthetic password in the background. This should
reduce the perceived latency of device unlock UI wise.

Bug: 68380673
Test: Measured time manually, unlock on marlin now takes 70ms, and taimen
      takes 90ms.

Change-Id: Ibb787251f8fea776c6745d45ee2d8ab60515a5f4