plugins/overview: Make screen edge toggle overview rather than cycle between modes

The user may have no usecase for the grid view mode, i.e. they may want
to activate overview, do their thing, and then return back to normal
desktop. However, with the current behavior, there's one extra step
(switch to grid mode) in order to go back to desktop.

In hindsight, we should have added different screen edge actions for
overview and grid modes. This can be done in 6.1.

BUG: 481335


(cherry picked from commit f77f5b23)