Release predict_back animaton leash in Launcher.

Release the animation leash after back_to_home animation done.

Flag: EXEMPT bugfix
Bug: 395035430
Test: Use the back gesture to trigger the back_to_home animation
multiple times and perform multiple random transitions, dumping the
Surface Flinger to check the predict_back leashes not remain in the
offscreen layer.

Change-Id: I7ed9dfd510e38bed28b51762df9976d33c4a5644