Enable ART source build.

This is the default when the make config step is run, but we need to do
it explicitly here since soong.variables is hardcoded.

Test: env OUT_DIR=out prebuilts/build-tools/build-prebuilts.sh
  on the aosp-build-tools branch
Bug: 188922057
Change-Id: I1f85c01b76a212b5cdd1332fb2445b184f4ad759
1 file changed