Added function to explicitly initialize a namespace

This change replaces lazy get-or-create logic for
linker namespaces with the explicit one.

ApplicationLoaders.getClassLoader(..) is now resposible for
the namespace initialization for PathClassLoaders.

Bug: http://b/27189432
Bug: http://b/22548808
Change-Id: Ife987c3ca1db33a47c20f363a5ed61512be8b5a7
(cherry picked from commit d047c925)