Avoid triggering StrictMode when closing a socket.

Originally, we called getsockoptLinger unconditionally, using ENOTSOCK
to recognize the non-socket case, but the cost of throwing an exception
is so high (both on dalvik and art) that we worked around that by doing
an fstat first. This was mostly fine, but fstat calls onReadFromDisk,
which isn't true for the socket case (which causes upset for threads
that are allowed to touch the network but not allowed to touch disk,
which Chrome uses, even though it's not immediately obvious to me why
that makes any sense).

(cherry-pick of d636777d.)

Bug: 4972558
Bug: 12133112
Change-Id: Ic0b89d1f181e436b4f43c1c04c1f706883ab84c0