Read product vndk libraries only when available

Current linkerconfig implementation tries to read vndk library list with
product vndk version even when the version is not specified. This change
will let linkerconfig load those files only when product vndk version is
available.

Test: m -j passed && Checked execution from cuttlefish
Change-Id: Ifb861d7c71a43ac77b27a6c29419b5f965d6097d
1 file changed