The implementation in Parser doesn't descend into sub-thread.
Re-use the getThreadSummary method in ThreadItem and traverse
the thread properly.
Bug: T298617
Change-Id: I318d9012eb83f37ccbe463923524ef2e9f995ced
The user interface only allows you to subscribe to level 2 headings.
But we would generate events for whatever heading was the closest,
If it was e.g. level 3, no one would receive that notification.
Now we generate events for the closest level 2 heading, or we don't
generate the event at all if there isn't one (if the only headings are
of level 3 and below, or level 1, or if the comment is added before
the first heading on the page).
Bug: T286736
Change-Id: Iae99853070e353ab81c9cc29ef1d53c877adfc66