Fix URLConnectionTest.testConnectTimeouts() by dropping use of Mockito

This test used a Mockito spy to wrap a Socket instance.

It has been failing on AOSP for (at least) weeks due to what
appears like a bug in AOSP's version of Mockito; this bug can lead
to crashes in <clinit>, or the expectations set up on the spy not
working correctly. There is probably some way we could call the
Mockito API to cause the static initializers to run in a different
order and work around the bug, but that would require more
investigation and would likely still be fragile. It doesn't seem
like a good trade-off as long as this only affects isolated cases.

Therefore this CL replaces the spy with an explicit DelegatingSocket
wrapper instead. This avoids the need to investigate a potentially
fragile workaround for the Mockito behavior and increases clarity by
removing the runtime magic.

Bug: 29095428
Change-Id: I46d57f18d9ea157e8a34ce3aead55e0f97de458a