Replace hidden getFileDescriptor$()

It's the preparation for the incoming ConnectivityService
mainline. The mainline modules can not access hidden APIs.
Thus, the hidden APIs usage should be removed.

The getFileDescriptor$() in Socket or DatagramSocket is
hidden for internal usage. It should be used in a module and
not prefer to expose as an API. Thus, replace it with
ParcelFileDescriptor APIs to get the fd via
ParcelFileDescriptor.

Bug: 170598012
Test: atest FrameworksNetTests
Change-Id: I05b31a48315e7e6d95e22123103e544a7136a449