Revert "Revert "Revert^2 "Add more variables for system/build.pr..." am: aaf50f6a

Original change: https://android-review.googlesource.com/c/platform/build/+/3196510



Change-Id: I4c9832cf04226e6c0aa33cfb544c70dc2a805ed2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>