Use shared variant of dep for packaging

For native modules that have both static and shared variants (e.g.
cc_library), the deps mutator of android_filesystem would always create
a dep to the static variant. This is likely due to the fact that
`AddFarVariationDependencies` creates a dependency on the first variant
of the dep which matches the requested variations. `static` appears
before `shared` in linkMutator, and therefore android_filesystem would
always create a dep to the static variant.

This CL uses `OtherModuleFarDependencyVariantExists` to create a dep to
the shared variant. If a cc_library is listed in `PRODUCT_PACKAGES`, it
always means the shared variant.

Test: go test./filesystem
Test: diff in kati install files of vendor/ before and after this CL
https://diff.googleplex.com/#key=qrY73chVkwff

Change-Id: Iea9d6fde199ef95d43da2c041e2f84e5a7951285