Allow sloppy touch targeting on Action Bar home/up

Bug 6521608

When there are no custom views or other actionable views next to the
action bar's home/up affordance, expand the touch target by a generous
slop value. This helps frustrating cases where a user accidentally
hits just to the right of Up and nothing happens.

Change-Id: I09ef604ce70ad380aab62a373b04c4b02007c644