Expose accept(2).

The implementation of PlainSocketImpl.accept is made ugly by the fact that the
SocketImpl we mutate may share its FileDescriptor with a SocketChannel, so we
need to mutate the FileDescriptor itself, and can't simply swap in a new one.
I've raised http://b/4452981 to see if we can integrate the nio and io socket
classes more closely, to avoid this kind of mess.

Bug: 3107501
Change-Id: I4964b64c25e936a44d1e4c22504ca29bba247ab6
15 files changed