Rename a few constants and clarify their meaning in comments. am: 2687277fd7 am: 4ba0234448 am: d64694fa35 am: 70a8ce1347

Original change: https://android-review.googlesource.com/c/platform/art/+/2229498

Change-Id: Ic34b5fc6f7bc03e55dbe91e7d6e68c1d6b38a9d1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>