Implement back navigation on communal hub

Navigating back on the hub will take the user back to the blank scene,
and close the hub.

Fixes: 346331399
Test: flashed and verified back gesture on right side of the hub
will close the hub, while left side doesn't trigger back gesture.
Flag: com.android.systemui.glanceable_hub_back_gesture

Change-Id: I86dc45a0c3fabd3c9f548bc21e18b1fc157ae3c5
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 07f7436..3f165a3 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -1052,6 +1052,16 @@
 }
 
 flag {
+  name: "glanceable_hub_back_gesture"
+  namespace: "systemui"
+  description: "Enables back gesture on the glanceable hub"
+  bug: "346331399"
+  metadata {
+    purpose: PURPOSE_BUGFIX
+  }
+}
+
+flag {
   name: "glanceable_hub_allow_keyguard_when_dreaming"
   namespace: "systemui"
   description: "Allows users to exit dream to keyguard with glanceable hub enabled"