make APEXes with public libraries visible

make APEXes with public libraries visible

Public libraries (listed in /system/etc/public.libraries.txt) should be
available to classloader-namespace (created by libnativeloader). So,
linkerconfig reads public.libraries.txt and makes apexes with public
libraries visible.  This cleans up hard-coded references to nn, i18n, art
apexes.

And the list of apexes along with public libs are stored in
/linkerconfig/apex.libraries.config.txt so that libnativeloader can read
it and link them to classloader-namespace.

Bug: 150767721
Test: cuttlefish boots
Test: atest --host linkerconfig_diff_test
Change-Id: Idd7e2b0bcc6e54cd79c2b9c4c755f883b4507b74