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

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

Change-Id: I2d0c4a0915e671905793d1cc5ed17143791d67f6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>