Fix flicker to OVERVIEW state during quick switch

Here's what was happening:
- When animating to a new task, the launcher controller ends first (when
  quick switching it likely ends immediately since we're already scaled up).
- Then we get onApplyWindowInsets(), which re-builds the launcher controller.
- Finally, the window animation ends, invalidating the handler and ending
  the new controller, which sets the state to OVERVIEW.

To fix this, never create a new controller if it has ever been started.

Bug: 133508173
Change-Id: Ibd0e18e488353df73159ffbdd9a3b335bed57d98