show all channels for updatable add-ons if a beta/dev add-on has been installed (#16637)
If a beta/dev add-on has been installed, updates for that add-on are not displayed by default when going to the updatable add-ons tab.
This is because the default channel view for updatable add-ons is always "Stable".
Instead, it would be helpful to show beta/dev add-ons if these have been installed previously.
Description of user facing changes
If a beta/dev add-on has been installed, add-ons of all channels will show up by default in the updatable add-ons tab
Description of development approach
Set the filter key depending on if beta/dev add-ons have been installed