Preload public libraries on Runtime::Start()

Preloading public libraries on Runtime::Start()
instead of doing it in native_loader c-tor
ensures that the libraries are loaded only for
binaries that are using Android Runtime, and not
just linking against libandroid_runtime or libart.

Loading public libraries on Runtime::Start()
also helps some apps to save startup time and space
by ensuring that all NDK libraries are loaded
and ready to use.

Bug: http://b/27245894
Change-Id: I15679a72450f3d18a7dd503d0ae977cebcfd7f52