Don't add cells to FocusLogic sparse matrix if they aren't focusable.

Previously, any child of ShortcutAndWidgetContainer was added to the
matrix, causing widgets (which aren't focusable) to be considered as
potential targets to gain focus when an arrow key was pressed. But if
the algorithm chose them, they couldn't take the focus so nothing
happened (i.e. the focus stayed on the app/folder it was on before).

Bug: 25126768
Change-Id: Id55fc310f7f58fb8795cce51dcefe4fd1210f788