Fix missing grouping after shade closes

Fixes: 356516285

Test: HeadsUpManagerPhoneTest

Test: send message HUN from two contacts from same app
      open shade, close shade
      => see that HUNs are grouped

Test: send delayed HUN, open shade, close shade before delayed HUN
      times out
      => see no disappear animation (no regression)

Flag: com.android.systemui.notification_avalanche_throttle_hun

Change-Id: Ib49522e57811b2670a60b3d05e92c3b7410cec87