Add focus ring to widgets header

Change the widgets header drawable to be a selector with the
state_focused property in addition to the state_expanded property. The
focused items are drawn with a focus ring with no ripple, while the
unfocused items have a ripple.

Bug: 365120324
Test: focus traversal through left widget picker pane
Flag: EXEMPT bug fix
Change-Id: I710cb9e2d727a061e0c19df15526e8c832f6366b