Make DownloadScheme.isAuthSupported an abstract method

It is too easy to forget to override isAuthSupported in download
schemes that support but do not require authentication.  Make it
abstract so anyone adding a new download scheme has to make an
explicit choice about whether it should show up on changes that are
not world-readable.

Change-Id: Ifbc70c0d3b6eea08ceee726317381cabd3892e03
2 files changed