[iOS] Adjust Keyboard Scrolling for Sticky Headers and Fix Bottom Content Inset

[tj-devel709]

Description of Change

This PR fixes two different issues regarding the iOS Keyboard Scrolling.

1. If there is a "Sticky Header" or some other view above the scrollview, sometimes, the content would not take that in account and scroll the entry behind this header. The fix for this includes calculating the size of the scrollview and compare that to the standard boundaries of the navbar and status bar on the top, and the keyboard on the bottom.

Before	After
StickyHeader_Before.mov	iOSStickyScrolling.mov

2. When scrolling occurs on an entry or editor, we are not able to scroll to the bottom of the scrollview when the keyboard is still open. The fix for this one was changing the previous calculation on the content inset bottom to the size of the keyboard. Sometimes, simpler is better :)

Before	After
BottomContentInset_Before.mov	iOSBottomInsets_After.mov

Issues Fixed

Fixes #19956
Fixes #20200

[mattleibow]

This PR only touches iOS. The failing Windows and Android testa re currently being fixed in separate PRs.