Clarify the format and assumptions on the dex file locations. am: 949c197585

Original change: https://android-review.googlesource.com/c/platform/art/+/2969535

Change-Id: I15da38f08ff479eafe745e6ce510abd4b3248963
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>