Rename PackageState getUsesLibraries to getSharedLibraryDependencies

This better communicates what the method returns and prevents it from
being confused with the <uses-library> manifest tag, which is only
one of the 5 types of libraries returned by this method.

Also fixes a null vs empty check calling this method.

API-Coverage-Bug: 262593427

Bug: 265006762

Change-Id: Ia7ab400cfdb4dd9a9ff66cb9c102a21e3f2b95cb