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: If275e249d2b46ee6cf22a06d62374bbf47c55342
diff --git a/api/current.txt b/api/current.txt
index 4201b6b..0ec2787 100644
--- a/api/current.txt
+++ b/api/current.txt
@@ -50416,7 +50416,7 @@
     method protected abstract void connect(java.net.InetAddress, int) throws java.io.IOException;
     method protected abstract void connect(java.net.SocketAddress, int) throws java.io.IOException;
     method protected abstract void create(boolean) throws java.io.IOException;
-    method public java.io.FileDescriptor getFileDescriptor();
+    method protected java.io.FileDescriptor getFileDescriptor();
     method protected java.net.InetAddress getInetAddress();
     method protected abstract java.io.InputStream getInputStream() throws java.io.IOException;
     method protected int getLocalPort();
diff --git a/api/system-current.txt b/api/system-current.txt
index 06e1ab8..73c2e33 100644
--- a/api/system-current.txt
+++ b/api/system-current.txt
@@ -53034,7 +53034,7 @@
     method protected abstract void connect(java.net.InetAddress, int) throws java.io.IOException;
     method protected abstract void connect(java.net.SocketAddress, int) throws java.io.IOException;
     method protected abstract void create(boolean) throws java.io.IOException;
-    method public java.io.FileDescriptor getFileDescriptor();
+    method protected java.io.FileDescriptor getFileDescriptor();
     method protected java.net.InetAddress getInetAddress();
     method protected abstract java.io.InputStream getInputStream() throws java.io.IOException;
     method protected int getLocalPort();