Move public libraries list to a separate source file

Functions for reading the public libraries list are moved out of
library_namespace.cpp to public_libraries.cpp.

In addition, library-local symbols are moved from android namespace to
android::nativeloader namespace.

Bug: 130388701
Test: build & pass presubmit tests
Change-Id: If82419598304d56d29bfec4ef553443c788d0f53