Update visibility of the stack dropdown list after the search view is collapsed.

Previously we hid the stack dropdown list just before the action bar is
collapsed. Then the dropdown list gets back to be visible when the action bar is
collapsed.

The CL updates visibility of the stack dropdown list just after the action bar
is collapsed.

BUG=20945584
Change-Id: Iaf5055bce101f8134accd311829e50f441ddc98b