addrelease.py: Use approximated "~length" for Recordings.

Recordings don't support sub-second time precision, so the
track.metadata.length used for the two Add...AsRelease classes will
cause an error on /recording/create's "length" field.
track.metadata["~length"] contains the time converted to mm:ss format
and will not cause the error on /recording/create, so this commit
changes AddFileAsRecording to use that.