Check vndk-private with the IsVndkPrivate()

*_available will not provide if a module is VNDK-private or not.
Fixed it by using IsVndkPrivate() with private() interface.

By allowing exclude_from_vendor_snapshot for vendor_available
modules, available() interface is no longer required. Tests are
updated for exclude_from_vendor_snapshot on vendor_available modules.

Bug: 175768895
Test: m nothing
Change-Id: I9ff85256170d59b378a4c354d5c007cfbeacabc5