linker: implement shared namespaces

Shared namespaces clone the list of loaded native
libraries from the caller namespace. This allows
classloaders for bundled apps to share already loaded
libraries with default namespace.

Bug: http://b/22548808
Bug: http://b/26165097
Change-Id: I8949d45937fdb38e1f586ff0679003adac0d9dad
(cherry picked from commit e78deef3)