Previously, only comments could have IDs, because we only needed IDs
for replying. But we might also use them for notifications soon.
Bug: T264478
Change-Id: I1bcad02bf17ab54bc5028a959543c10f0430836b
* Pass rootNode to the constructor
* Rename getters to match CommentItem/HeadingItem/ThreadItem
value classes.
* Always build the thread tree so CommentItem's always have
and ID and replies/parent.
Change-Id: I508be9534de59016ff806e3d84edcbb1c76cb0c6