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

(cherry picked from commit 4f263fdf)

Change-Id: I6746f4e0e5a6c9788cdcf35f74ccca5aa9a0e8b4