Fix and clean up FileDescriptor.isSocket.

The original logic was accidentally inverted.  This prevented sockets
from being untagged, which would result in cancelled requests stalling
out with sub-optimal TCP ZeroWindow detection instead of sending RST
packets.

Add a comment explaining how this works.

Switch from SO_ERROR (which has a side-effect) to SO_DEBUG (which doesn't).

Add a unit test.

Bug: 18706592
Change-Id: Ib822a9727668908e04f87a6deb104dcebf333f05