SocketImpl#getFileDescriptor shouldn't be public

It's protected, as it was before openJdk switch. Added
hidden public method getFD$ for tests.

Bug: 26689841
Change-Id: I7e4149a5100b448e8729536f9718a35707a1c290
(cherry picked from commit 5dfeb54cb55eab53e5307e205a16b59e1da5ab07)
3 files changed