Migrate `MDCBottomNavigationBarColorThemer` logic to theming extensions to prepare for the eventual deletion of `MDCBottomNavigationBarColorThemer`.
Part of #9130
We need to add @objc annotations to colorScheme and typographyScheme instances in our Swift examples, because we moved to Swift 4.2, the respondsToSelector won't find `setColorScheme:` and `setTypographyScheme:` setters otherwise.
This is a follow up PR for #7166 adds @objc annotations to Swift catalogMetadata() methods, because the Swift 4 compiler no longer attempts to infer what methods should be visible to Objective-C. As a result of this change in the compiler, no Swift examples were showing up in Dragons after #7166. See this article: https://useyourloaf.com/blog/objc-warnings-upgrading-to-swift-4/ for additional context.
Adds support to BottomNavigationBar to more easily work with Autolayout.
* `intrinsicContentSize` returns the correct size of the bar items as content.
* `barItemsBottomAnchor` allows anchoring the Bottom Navigation bar to the safe area layout guide.
Updates the `MDCBottomNavigationBarController` to use the new layout anchor and eliminates the need to respond to safe area changes in the view controller.
Fixes#4520
Adds constraints safe for use on iOS 8. This removes the availability
checks previously set on the MDCBottomNavigationBarController class that
may have been causing CocoaPods lint errors.
Related to #6608
