Skip stash animation when destroying LauncherTaskbarUIController

Follow up to ag/28978773
It seems like mControllers == null doesn't work when folding. The launcher visibility change still triggers the stash animation from onLauncherVisibilityChanged
by LauncherTaskbarUIController#onDestroy

Bug: 362713428
Test: manually make sure that the stash animation isn't triggered from folding
Flag: EXEMPT bugfix
Change-Id: I04539e7a31d6c329eacb54946e3b3c211e47a709