Scroll to the selected widget picker row after headers are clicked

This keeps the row in view. Currently, this will scroll the row to the
centre-bottom of the screen due to the top padding being double counted,
but that will remedied in a future CL.

This also resolves the issue where the last row's widgets aren't visible
by handling that case specially

Fix: 188665456
Bug: 183378651
Test: verified locally
Change-Id: I9acb9087a8cdaf130ac5955c810c96462b368f36