[Flexiglass] Fix 2-finger swipe on single shade to go to Quick Settings.

Bug: 359173565
Bug: 356596436
Flag: com.android.systemui.scene_container
Test: Added unit tests.
Test: Existing unit tests still pass.
Test: Manually by performing the gesture and verifying QS is opened on
 a folded device, and the split shade on unfolded device.
Change-Id: Ie54fa74228d1f426ff6f5b0982ec8bfe23061007