Cleanup copyDocument and moveDocument in SAF.

1. Unhide the methods in DocumentsProvider.
2. Stop revoking grants, as we shouldn't do that if the documentId doesn't
   change. Note, we could have an if checking if the ID changed then revoke
   in call(), but it would be racey, as another file with the same ID might
   have been created in the meantime on another thread.

Change-Id: Ia99853bf11cf416e0b8c0a0e63458dcc09564da9