Simplify LoadNativeLibrary()

With ApplicationLoaders.getClassLoader() ensuring
linker-namespace initialization there is no longer
need for LoadNativeLibrary() and callers to pass
along namespace-specific information to art.

This change removes unnecessary parameters of such
calls.

Bug: http://b/27189432
Bug: http://b/22548808
Change-Id: I341d35a2d5195e634678b352f4361f8712984b69