Notify routers when the suggestion space is visible

Part of un-reverting ag/32935663. Notify the router when the suggestion
space becomes visible, to allow suggestion providers to provide
suggestions.

Test: atest
Bug: 409098190
Flag: com.android.systemui.enable_suggested_device_ui
Change-Id: I9f001b3d4220a02df257ef383f382e46167d3d92