linker: Allow caller to specify parent namespace

This change enables apps to share libraries opened
with RTLD_GLOBAL between different classloader namespaces.

The new parameter to create_namespace allows native_loader
to instruct the linker to share libraries belonging to
global group from a specified namespace instead of
using the caller_ns.

Bug: http://b/28560538
Bug: https://code.google.com/p/android/issues/detail?id=208458
Change-Id: I5d0c62730bbed19cdeb16c7559c74aa262a2475f