Tighten bootclasspath_fragment property validation

Previously, due to legacy reasons, the property validation did not
require a contents property and allowed the image_name to be either
"art" or "boot". Those reasons no longer apply and so this change
requires a contents property and only allows the image_name to be set
to "art" if specified.

Bug: 177892522
Test: m nothing
Change-Id: I8855d6e5365ef0b55490e90e7b6c0081cf070ee5