CMUpdater: select default update type if an invalid one is provided

In the case when users had the *removed* update type (array index 3), we
were not selecting any channels for the outgoing JSON request. So select
the snapshot channel by default.

Change-Id: I75b91f9d6d57d45ee417235b28e5709fc5a60f6a
Signed-off-by: Roman Birg <roman@cyngn.com>