Add support to indicate bitness of public library.

For public vendor libraries, a vendor might have only a 32 bit
or only a 64 bit version of the library. Add a way to indicate this
in the public.libraries.txt files. The new format is:

  library.so 32

This indicates that this is a 32 bit only public library.

  library.so 64

This indicates that this is a 64 bit only public library.

Bug: 29370721
Bug: 29512261
Change-Id: I26b14893d104a9e1f62914663d8c972cd4d55e1e