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
(cherry picked from commit 6664a805)

Change-Id: Id52c8b61d5c802ce62edda7e25f2755f6f93d6b7