Previous code was inconsistent about how and when the FAB was hidden if
the user had set the relevant preference.
- Sometimes the FAB has hidden by setting the visibility to false, which
removed it with no animation.
- Sometimes the value of the preference was checked once, when the
fragment or activity was created.
- Some timelines didn't show the FAB (Hashtags, Favourites, Bookmarks,
TrendingLinks, TrendingStatuses).
- Logic for figuring out which `ComposeActivity` intent to use was
scattered across different files.
Improve this by:
- Expose changes to the `FAB_HIDE` preference in the relevant viewmodels
as a flow the UI component can collect.
- Centralise the show/hide logic in a new `ActionButtonScrollListener`
class, and always using `show()`/`hide()` to animate the transition.
- Centralise the logic for creating the `ComposeActivity` intent in
`TabViewData`.
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