Fix Os.sendfile() for null offset.

A previous CL ( http://r.android.com/518135 ) introduced a bug where
android.system.Os would throw NullPointerException when given a null
offset argument, rather than treating it as an offset of zero.

This CL fixes this, and also introduces test coverage for
both sendfile() in both android.system.Os and libcore.io.Libcore.os.

Test: OsTest fails before this CL but passes after this CL, as well
      as on Android 7.1.
Change-Id: Ie39bb9a8f209004109ad82cb5ea56c9b7ab74cbf