Avoid showing lockscreen when closing an activity in hub mode.

When the glanceable hub is active, we want to avoid showing the
lockscreen. Currently, the lockscreen wallpaper is visible when swiping
away an activity - since the activity is y-translated upwards.

This fix allows systemui to set an opaque background for the recents
transition. We then set the background to be the same color as the hub.

Test: atest WmShellTest
Bug: 332591536
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I82e8a1d66a951b9b9fabdbec4f69d3a91242c30b