New trace slice for start of user switch

Add new trace section: UserDetailView.Adapter#onClick. This makes it
easier to spot the start of the user switch. Eventually, this may be
used in perfetto metrics for measuring total user switch time.

Bug: 197028912
Test: Capture perfetto trace, search for UserDetailView.Adapter#onClick
Change-Id: I38bebd27710123b53a0b840c523781773427eddb