Add bootclasspath_fragment as an alias for boot_image

This is part of the work to rename boot_image to bootclasspath_fragment
which is being done for two reasons:
1. To avoid clashing with the bootimg module type.
2. To better reflect what this represents.

While a bootclasspath_fragment can create what ART calls a boot image
(which is different to what the bootimg module type represents) it does
not have to do so.

Bug: 177892522
Test: m nothing
Change-Id: Ib45604be7adc790ded9e27a2ac812dd7522ca8db