Add hack to make sure Socket#impl field is not null

Conscrypt's compat code inspects the "impl" field of Socket and expects
it to be non-null. However, SocketChannel creates an adaptor that sets
the impl field to non-null. This can't be fixed immediately, so put this
in to maintain the old behavior.

Bug: 25857624
Change-Id: I2d79a6d9a77d9102203cfb43df09d70aff99ffe3