Use LayoutManager.onLayoutCompleted as a signal that content has changed

onChildAttachedToWindow is called before the layout manager is finished
processing the layout, so incorrect measurements / intermediate
animation states can be present.

onLayoutCompleted occurs after each full layout calculation, including
measuring multiple views or animating, and is a better signal to use.

Fix: 190391539
Test: verified locally with search use case and other expand collapse
cases

Change-Id: If01caf33c6c3371636895e1361d80de536363239