• bring back keyboard frame tracking to OWSViewController if running on iOS 15.
• add our own layout guide to OWSViewController that is updated based on keyboard frame notifications on iOS 15; on newer iOS versions it's a proxy to view.keyboardLayoutGuide.
• use OWSViewController.keyboardLayoutGuide where needed.
• in media composer screen (not OWSVIewController subclass) we already track keyboard frame - use that to position bottom toolbar on iOS 15.
I have also discovered that profile avatar editing screen was somewhat broken on older (15, 16) iOS versions. Fixed that here.
