Allow setting PRODUCT_ENFORCE_ARTIFACT_PATH_REQUIREMENTS to false

Some products clear PRODUCT_ENFORCE_ARTIFACT_PATH_REQUIREMENTS
in order to disable it. But oftentimes this clearing relies on
the fact that clearing a variable will also cause it to not
inherit from any already-inherited files. Starlark doesn't support
this inheritance behavior, so allow setting
PRODUCT_ENFORCE_ARTIFACT_PATH_REQUIREMENTS to false so that it
can be explicetly disabled instead of relying on the clearing
behavior.

Bug: 221312707
Test: Manually
Change-Id: I03a227911c6985a65fc5d210682bbd4ac9bd6747