Revert "Revert "Expose correct FileDescriptor in DatagramSocketAdaptor""

This reverts commit 891ab5193d1ccb28bf342a8026a6b421ac96cd9b.

The function was erroneously added to the API list because it used @hide
annotation inside a comment. @hide annotation is only effective in
Javadoc.

Bug: 26292854
Bug: 26689841
Change-Id: Ib85346b3afceb945fbd12cf52cda0c28dd06da3c
4 files changed