Hide floating tab bar on iPad when scrolling

[mazevedofs]

Phabricator: https://phabricator.wikimedia.org/T383759

Notes

• Works on current UIKit implementation

Test Steps

1. Scroll up and down on Article View, Explore, Saved, Search and History.
2. Make sure it behaves as expected
3. Check if iPhone is not affected

[tonisevener]

I notice the tab bar can get "stuck" in a hidden state when the content is short. I faced a similar issue with the navigation bar hiding - you can reference calculateNavigationBarHiddenState for how I fixed. Not a must-fix though, in my opinion, as long as design review is good with it.

in.mov

Per design, we need a bug fixed so that the floating tab bar is not stuck in a hidden state.