Adding bootimage_test_harness artifact to droidcore

Currently bootimage_test_harness is only built on a few targets
on release branches. However, we might have a last minute build
breakage on release branches as bootimage_test_harness is not
built on dev branches. Moving it to droidcore so we can have more
completed build coverage on pre-submit/post-submit builds.

Bug: 183865297
Test: `make` then check $OUT
Change-Id: I496609401a9444706726256d2f7b45c6bb69ee94