Revert^4 "Package dexpreopt artifacts for libcore jars in the ART apex."

This reverts commit bf0e4764.

Reason for revert: coverage build with EMMA_INSTRUMENT_FRAMEWORK=true
is fixed by inspecting the environment variable and not generating
boot image in case it is set.

Dexpreopt artifacts for the libcore part of the boot class path are
now packaged in the ART apex. The system image still contains
dexpreopt artifacts for the full set of boot class path libraries
(both libcore and framework); the libcore part will be removed and
boot image extension will be used in a follow-up CL.

Since this is specific to the ART apex and makes no sense for other
apexes, the implementation adds a boolean flag "is ART apex" rather
than a new apex module property.

Build rules for the new set of dexpreopt artifacts are created using
a new variant of the global boot image config. Previously we had two
variants: "default" (for the system image) and "apex" (for the
JIT-zygote experiment). This patch adds a third "art" variant.

Test: m
Test: m art/build/apex/runtests.sh

Bug: 144091989
Change-Id: I113c0d39222d6d697cb62cd09d5010607872fc2b