applets/taskmanager: manually set hover: true on the tooltip highlight

Since we're manually managing appearance and disappearance rather than
letting the Highlight component take care of it for us, we need to make
it always be visible so that when we do show it, it's got the right
appearance.

Note that unlike the original commit, here we set hover: and not
hovered: because in Plasma 5.24, we were still using the old
PlasmaComponents2 version of the Highlight, which only had hover:, and
not hovered:

(cherry picked from commit ed6a9d1c)