Set parent namespace for linker-namespaces

This change allows applications to share RTLD_GLOBAL
native libraries between namespaces associated with
different classloaders.

The rule is - if a library is GLOBAL within namespace
associated with parent classloader, it is shared
with namespace associated with this classloader

Note that the sharing happens on create_namespace
event, which is tied to createClassloader in case of
application classloaders created by the framework, for
custom application classloaders it is tied to first
loadLibrary() event.

Bug: http://b/28560538
Bug: https://code.google.com/p/android/issues/detail?id=208458
Change-Id: I7ee701166f8ec5eff033b7acc0f80c7aa4ec5bda