Add ui event for auto confirmation attempt

We want to keep track of how many unique users are using the auto
confirmation feature. We will add this event tracking here so that we
can perform this measurement.

See go/bouncer-auto-confirmation-event

Test: check aster dashboard
Bug: 313941957
Flag: NONE
Change-Id: I0fa14b4fd1ef6812034d023766e966c566229e91