Start fingerprint authentication in onStartedGoingToSleep

We used to start fingerprint authentication in onFinishedGoingToSleep.
This was a UX issue because then users couldn't place the finger on
the sensor immediately after pressing the power button because
onFinishedGoingToSleep is significantly delayed (around 900ms after
pressing the power button).

Bug: 23570959
Change-Id: I0bf557ebd10e6a8b033ab98a78aa338bf6538dcc