Ignore updates to bubbles that aren't from the system

The new notif pipeline code adds a concept of updates from the system
vs from sysui. Currently bubbles is responding to all updates but
I think we really only care about updates from the system (i.e. from
NotificationManagerService).

Bug: 238896626
Test: atest BubblesTest#testNonSystemUpdatesIgnored
Test: atest --no-bazel-mode --iterations 20 PlatformScenarioTests:android.platform.test.scenario.sysui.bubble.ExpandAndDragToDismissTest
      (with vadim's CLs and a 2nd change to be uploaded)
Change-Id: I3ffd6eb0f0a07448b2117924be514347c6e80b16