Added DropdownButton to the widget catalog

[awazgyawali]

Addresses this issue flutter/flutter#14460
We might need to add "Spinner" word somewhere to make Android developers easy to find the widget.

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[awazgyawali]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[Sfshaza]

@chalin, can you take a look? I know this PR is against the master branch, but we'll need to add it to the new branch as well.

[Sfshaza]

"A input field to choose..." => "An input field for choosing..."

I thought the idea was to add Spinner to this description for Android users?

[Sfshaza]

Why a placeholder image?

[sfshaza2]

Nevermind, we use lots of them.

[chalin]

Given that this is not a P0, @awazgyawali could you close this PR and reopen one over the dash branch?

[awazgyawali]

OK @chalin

[chalin]

Thanks!

[MarkOSullivan94]

Has this been merged in?

[chalin]

No, but #1903 was merged and it added DropdownButton.