Samples should use target sdk from FrescoConfig (#2522)
Samples should use target sdk from FrescoConfig (#2522)

Summary:
Thanks for submitting a PR! Please read these instructions carefully:

- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [x] Match the **code formatting** of the rest of the codebase.
- [x] Target the `master` branch

## Motivation (required)

What existing problem does the pull request solve?
The samples provided should use the minSdkVersion provided in the config file
The "showcase" sample currently has the minSdkVersion pinned to 21

## Test Plan (required)

A good test plan has the exact commands you ran and their output, provides screenshots or videos if the pull request changes UI or updates the website. See [What is a Test Plan?][1] to learn more.

Verify that the "showcase" sample works after the change.

If you have added code that should be tested, add tests.

No extra unit tests.

## Next Steps

Sign the [CLA][2], if you haven't already.

Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

Make sure all **tests pass** on [Circle CI][4]. PRs that break tests are unlikely to be merged.

For more info, see the [Contributing guide][4].

[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: http://circleci.com/gh/facebook/fresco
[4]: https://github.com/facebook/fresco/blob/master/CONTRIBUTING.md

Pull Request resolved: https://github.com/facebook/fresco/pull/2522

Reviewed By: oprisnik

Differential Revision: D23074607

Pulled By: wizh

fbshipit-source-id: 0ccf206878e056c8936b80db3bf48c508f526306

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