New download manager error code when we can't resume.

Adding DownloadManager.ERROR_CANNOT_RESUME for when we can't resume an
interrupted download (because the server didn't provide an ETag or
doesn't support range requests), as well as the necessary private
status code in Downloads.Impl.

Change-Id: I5a66f1e1964198552ab2216aa6d3cc0db2254e21
3 files changed