Remove dangling links in secondary namespaces

linker didn't remove link to the soinfo from shared
namespaces on soinfo_unload, because it didn't keep
record of all namespaces the library is added to.

This change adds test for this and also fixes the
problem by introducing list of secondary namespaces
to soinfo, which is used to remove soinfo in
soinfo::remove_all_links().

Bug: http://b/28115950
Change-Id: Ifbf6e54f92fa6e88f86b6a8dd6dc22d4553afd22