Add flag to gate checking last task visibility on dismiss.

Bug: 404203362
Test: Manual. Open 9 apps, dismiss all bottom grid tasks, dismiss first top task, observe no jump.
Flag: com.android.launcher3.enable_expressive_dismiss_task_motion
Change-Id: Ie21d234f6a44c4566c4f73d4cc3c32f6a8375e6d