Fix Dropdown componentWillReceiveProps to handle setting of state correctly

[mlmassey]

I noted when reviewing issue #2375 that the Dropdown was incorrectly calling setState() multiple times in the componentWillReceiveProps. The main issue is that the functions setValue() and setSelectedIndex() both operate on this.state, even though this.state was not updated correctly by the AutoControlledComponent prior to calling them. This fix addresses this by allowing a callback to be invoked to set further state after the AutoControlledComponent has called setState.

yarn test was run with full tests passing.

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Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

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