Create linker-namespace for the classloader

With this change ApplicationLoaders.getClassLoader()
creates linker-namespace for the classloader at the
construction time. Before this change the namespace
was created on first load of a jni library.

With this change we ensure that every classloader
has initialized namespace associated with it.

As an additional advantage we now can avoid storing
namespace-specific fields in the classloaders.

Bug: http://b/27189432
Bug: http://b/22548808
Change-Id: I3b160bd478a55171008682c40b2ebc13bdbd9882
(cherry picked from commit a55c7f15a3c386ec643f21eeb2f4e9fd7b03ba70)