Add alias variations to linkageMutator

Alias the shared variation if it exists, otherwise the static variation.
This allows modules that are not aware of shared library variations
(like cc_genrule) to depend on shared libraries by depending on
just the normal image, os and arch variations.

Bug: 162437057
Test: TestLibraryGenruleCmd
Change-Id: Icec57d43538e01ab05cc50d4e3f9a11cc55f0162