Store scopeToCancel.

Without this the scope that gets cancelled can be the new one after re-attaching. This results in no flows working - hence blank thumbnails.

Fix: 369590189
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: Manual - visually verified
Change-Id: Ie930d890095c30c4be8e87c795a06383ac06ea9e