closes#7990
This API change enables more flexibility on the text configuration on banner. Minor visual difference was generated by UITextView's implementation.
Previously, `intrinsicContentSize` was not supported in MDCBannerView, which means users have to specify layout constraints for width and height manually. After adding `intrinsicContentSize` support, users will only need to specify width constraints for banner to show it properly. This implementation and behavior matches UITextView.
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