Unlike most module types, config variable handling is always namespaced
to the Android.bp file, which limits reuse of the variable definitions.
Additionally multiple of these module types can define a string variable
in the same config namespace, but specify different valid values for the
string.
Previously, we cached the first instance we see of variable + namespace;
however, this caused non-determinism in which defintion would be used
and not migrating all values. Instead, we now only cache within a single
Android.bp file where the variable definitions are re-used.
Test: go tests
Bug: 271481817
Change-Id: Ic327657c508e47a705bacd24712a1916e105c7cd
8103dc428e