Create dedicated bugfix flag for vertical swipes.

This changelist introduces a new flag to track vertical swipe
behavior in hub mode as a bugfix.

Test: manual - ensured new flag is still used.
Fixes: 340177049
Flag: com.android.systemui.hubmode_fullscreen_vertical_swipe_fix
Change-Id: I1455077edcf32cae539a82abc1fa82ae26af4875