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

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

Change-Id: Iba6c667bf86c48fe9f27d8bbf154f79144a4630d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>