Include shared lib in vendor snapshot if isVndkExt

A VDNK extension is an image:vendor module provided by a
vendor-modified framework project. So it should be
provided to the vendor build as a prebuilt (for the
purposes of building against a vendor snapshot).

Exempt-From-Owner-Approval: cherry pick from internal branch

Bug: 160189878
Test: manual
Change-Id: I3eb4794c1be2949b9c85fd52f823e5e14df4ad7d
Merged-In: I3eb4794c1be2949b9c85fd52f823e5e14df4ad7d