Don't allow apps to request scrolls to out-of-bounds positions

An app was requesting smooth scrolling to a view position beyond the
number of items in the list. This caused our setup logic to execute on
every frame, waiting for the target view to be added.

This fix clamps the requested target position to the number of items
actually in the list.

Issue #6572175 Messaging: Sometimes conversation doesn't scroll when focus is brought to the compose field

Change-Id: I23707aeb213e67af4297713a03c2f5b446c8e2b6