Changing TextEditingController leads to out-of-sync controllers

[Hixie]

When you change the TextEditingController of an EditableText, we neither pull the new data from the new controller (which I guess is what we should do) nor push our data to the new controller.

cc @cbracken

[cbracken]

Added a test in #12276 to verify we don't regress this.

[github-actions]

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