[Desktop context menu] Hide Activities menu item when not using Activities

This is something that came up in the monthly VDG meeting: the desktop context menu has a lot of stuff in it, and we're looking to slim it down where possible so it isn't so overwhelming.

One potential target is the Activities menu item, which has no real use for a user who isn't using activities. We can never be 100% sure, but if there is only one activity, it's a good bet the user isn't using the feature. In this case, we can hide the menu item that shows the switcher, because there's nothing to switch to!

Now, the switcher also has a button to add new activities, so it could be argued that this change makes activities less discoverable. However the feature is shown pretty prominently in the commonly-accessed Workspace Behavior group in System Settings, so I think it's already discoverable enough.

Test plan:

• Have one activity and right-click on the desktop > See no menu item
• Create a second activity and right-click on the desktop > See it

Here's how the menu looks now when there is only one activity:

cc @teams/vdg @ivan