applets/notifications: use `Plasmoid.self` in `Globals.adopt`

Plasmoid will become an object in QML after the system tray is deleted,
but we need `!Plasmoid` to return true in order to delete the plasmoid
item in `Globals.plasmoids`, so use `Plasmoid.self` instead.

BUG: 459249
FIXED-IN: 5.26
(cherry picked from commit 0fb1453f)