Inbox: Reduce some scrolling jank by making one StickyHeaderListView item per topic / DM conversation

[chrisbobbe]

Instead of having a whole section (all-DMs or a stream) be an item.

Quoting Greg from #381 (review):

As I mentioned in our call today, I played with this on my Pixel 5 to see the performance, using a test user on chat.zulip.org who has a ton of unreads. In debug mode the performance was pretty choppy — occasional pauses while scrolling that I believe reflected a new section scrolling into view, so that with the current version the whole section has to be built and laid out, which can be hundreds of items for different topics. Those pauses in debug mode were sometimes around a second long.

It's fine for debug mode to be choppy, though, so I tried it in profile mode. There there's still enough jank to be noticeable, but it's a lot milder — mostly a couple of dropped frames at a time, sometimes like 150ms. So this level of performance is fine for Beta 1, but it's something we'll want to revisit for Beta 2.

Probably that means I'll rework something in StickyHeaderListView to make it possible to have a separate list item for each conversation, just like you did in a previous draft, and without the glitchy interactions that that led to between one section's header and the next. (Effectively the current StickyHeaderListView assumes that each header is no taller than the item it belongs to; that's true in the message list, and it's true here by making each entire section into an item, but it's not true if each conversation is its own item.)