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: I18f53356a108c1f7e0525c6f2f8130eac173e81e
(cherry picked from commit ce41c4ee22dbf4d53c2ba5a21a63a0a3081bd732)
3 files changed