Add libcore.net.NetworkSecurityPolicy.

This hidden libcore API is to be used by bundled network stacks to
determine whether cleartext network traffic is permitted. These stacks
cannot directly use the public Android framework API
android.security.NetworkSecurityPolicy because the framework package
typically compile-time depends on the packages offering these bundled
network stacks.

As an example, this CL makes java.util.logging.SocketHandler and
platform-provided URLConnection instances for the FTP protocol honor
this new flag.

To avoid duplication, the intention is that framework's
android.security.NetworkSecurityPolicy implementaion will switch to
this new libcore API to store/retrive the state of framework's
NetworkSecurityPolicy class.

Bug: 19215516
Change-Id: I78e348458975d69c52918582a43db25df87e2a15
4 files changed