This PR graduates ContainerScheme to ready.
This includes updating the podspecs, podfile, all the import statements related to ContainerScheme, updating .kokoro rewrite rules, and finally the readme to not have ContainerScheme regarded to as being in beta.
Ran locally kokoro with -b bazel successfully.
Resolves#6732
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.
## Introduction
Before we move container scheme to ready we need to have both the `colorScheme` and `typographyScheme` nonnull. By making colorScheme and typographyScheme of MDCContainerScheming nonnull, clients will reduce the amount of conditional checks required in their apps and reduce a category of potential errors (accidentally passing nil to a nonnull parameter).
## The problem
Both `colorScheme` and `typographyScheme` were nullable in the container scheme making it harder for clients to use the container scheme throughout their app on non Material UIElements, i.e. UIView.backgroundColor
## The fix
Make both of these properties nonnull.
## Additional work
- [x] I had to update the swift examples to not be optional and I additionally removed the nil checks from the ObjC examples.
