Generate fake versions of modules excluded in directed vendor snapshot.

When using the directed vendor snapshot, the build fails because soong
fails to find some variants of the modules that have been excluded
from VENDOR_SNAPSHOT_MODULES, even though those modules are not going
to be used by the build.

The solution implemented here is to generate fake versions of those
modules (empty files) and include them in the generated Android.bp,
so that soong finds the modules, even though trying to use them would
fail.

Bug: 171821997
Bug: 179275601

Test: source build/envsetup.sh
Test: m -j nothing

Change-Id: Ibd3e963ab3e5504c0ac817f7cabbd241bf47a5cb