ade: Use decode() function to bytes object

`subprocess.PIPE` returns a bytes object which has no attribute
`read`.

Signed-off-by: Justin Kim <justin.kim@collabora.com>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5358