libtaskmanager: Fix appmenu caching

The appmenu properties are not propagated to
the cached `KWindowInfo` due to missing window flags.
Forcing the creation of a new `KWindowInfo` with the correct flags
allows the appmenu to be displayed without requiring any refocusing.
Also fixes the caching for activities.

BUG: 422786


(cherry picked from commit 94a04a55)