Only listen to edge swipes to/from hub mode

This makes SceneTransitionLayout only listen to swipes from the right
edge when transitioning to hub mode and left edge when leaving.

A blank spacer is added above the LazyHorizontalGrid in order to block
touches so that the SceneTransitionLayout can receive them.

Unfortunately this doesn't automatically allow other inputs to pass
through so bouncer swipe still doesn't work.

Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Bug: 308813166
Test: manually tested on device
Change-Id: I9176c297f78417422ee84af2e64114a8a32e81e5