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

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

Change-Id: I775374929934ab2308209db1af630810b10a1505
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>