Make StatusBarNotification#mContextForDisplayId thread safe

The context can be get also from background threads. So it was possible that clearPackageContext was clearing the mContextForDisplayId while a new context was being created, resulting in a ConcurrentModificationException

Bug: 362719719
Bug: 390567645
Test: StatusBarNotificationTest
Flag: com.android.systemui.shade_window_goes_around
Change-Id: Iea03b2d0c42641d91589b7383c0bd5ee542433a7