Use isolated namespaces for app native libs

Linker namespaces provide necessary level
of isolation for application native libraries.

The native libraries will no longer be able to
mistakenly depend on platform private libraries
like /system/lib/libssl.so

This change creates one namespace for each instance
of class-loader and uses it when loading native libraries.

For backwards compatibility with older apps we keep
using default namespace and LD_LIBRARY_PATH if target
sdk version is <= 23. (currently set to 0 for testing)

Bug: http://b/22548808
Change-Id: I64e97af7450fbf7e3740ccddda96bb2f7c52e03b