Add new setting to Settings.Secure which controls hub mode enablement.

Follow-up change will add the control in Settings to actually flip this
setting on/off.

Test: Locally on device
Change-Id: Ib3215c79efd9fc4e5c30ea7f261b5a12c80ebacd

core/java/android/provider/Settings.java

@@ -10220,6 +10220,13 @@ public final class Settings {
         public static final String DEVICE_STATE_ROTATION_LOCK =
                 "device_state_rotation_lock";
 
+        /**
+         * Control whether communal mode is allowed on this device.
+         *
+         * @hide
+         */
+        public static final String COMMUNAL_MODE_ENABLED = "communal_mode_enabled";
+
         /**
          * These entries are considered common between the personal and the managed profile,
          * since the managed profile doesn't get to change them.