Make public Network#bindSocket(FileDescriptor).

Code that uses android.system.Os to create sockets as FileDescriptors
should be able to bind them to networks.  Note that FileDescriptors
could already be marked as "protected from VPNs" via
NetworkUtils#protectFromVpn(), but heretofore were not easily bound
to any particular network.

Bug: 21449922
Change-Id: I4bb86db5d95d5a55bb2d7e245848d11eaa351e65