[Unfold transition] Update latency tracker to listen for animation start

We are going to introduce potential delay before
starting the animation, so updating the latency
tracker to track the end of the unfold display switch
only when the animation is started.

Bug: 258214245
Test: atest com.android.systemui.unfold.UnfoldLatencyTrackerTest
Test: manual unfold with/without animations => check that metric is reported
Change-Id: If3017719631f115579f16493136cce0d0e5e2f80