Mark many single value variables as such

This changes these variables to not inherit multiple values,
which allow for some more flexibility in how/where they're defined
in makefiles.

Also remove the previous equivalent solution that was in place for
the PRODUCT_BUILD_* flags.

Bug: 116769560
Test: presubmit
Change-Id: Ia27a11d95263ceb45ffa355e75bb2aa6d9672139