Migrate overlapping config var defs in all bp file

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