Don't install modules in unexported namespaces

Make builds handle modules with duplicate names in different
namespaces by passing a list of exported namespaces to Soong.
Soong-only builds can try to install both modules, leading to
a duplicate rule error.  Re-use the same list of exported
namespaces in Soong, which will be empty in all existing
Soong-only builds.  This will effectively only install modules
in the root namespace in Soong-only builds.

Test: m checkbuild
Bug: 79369665
Change-Id: Ie80d4b77e1dce165a33579fcf58b571989794b35