Bugfix: Fix update channel dropdown

When switching to the beta update channel in testpilotcloud, a warning will
pop up. When canceled, the selection should reset. This did not work reliably
in localized clients due to the use of string comparisons. Using a numeric
index to keep track of the old value fixes the problem.

https://github.com/owncloud/client/issues/10251
https://github.com/owncloud/client/pull/10609
https://github.com/owncloud/client/pull/10627
