build: Explicitly include ART's make logic from art/art.mk

To prepare for SOONG_ONLY builds, this change moves away from the build
system's automatic discovery of `art/Android.mk`.

This updates the build system to include ART's legacy Make logic from
its new, renamed location at `art/art.mk`.

The inclusion is done in `main.mk` to ensure that ART's legacy targets
(especially test targets like `art-host-tests`) are globally available
for any `make` invocation, not just for specific product builds. This
centralizes the inclusion and makes it easier to manage and eventually
remove.

This change depends on the corresponding rename of `Android.mk` to
`art.mk` in the platform/art repository.

Bug: 396022230
Test: lunch && m build-art-host build-art-target
Change-Id: I24586d7f74f20698d5e86b8c5fe27edcc3506d86