plugins/overview: Allow switching between modes using shortcuts while already active

Overview and Grid modes have shortcuts assigned to them. While they
provide a way to toggle the overview effect between on and off state, in
other words overview <-> off or grid <-> off, it's not possible to move
between the modes by pressing those shortcuts, e.g. off -> overview ->
grid -> overview -> grid -> ... -> off.

The culprit seems to be that EffectTogglableState has two "inactive"
states - Inactive and Stopped. It's counter-intuitive and needs a further
cleanup.

To make switching between overview modes work, this change makes
EffectTogglableState::toggle() toggle the state based on the Active state.
There's only one active state.

CCBUG: 481335


(cherry picked from commit fc92544c)