Move android_namespace_t to a separate file.

Breaking up huge linker.cpp into smaller peaces
in order to make it easier to extract part of the
code that belongs to libdl.so and remove parts of
the code that do not belong to linker

(refactoring part 1 of many)

Change-Id: I57ac36677a815800dc127c8c45c3ea806c37e247
Test: bionic-unit-tests --gtest_filter=dl*:Dl*