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, NOT a "stable" branch.
What existing problem does the pull request solve?
* Combined repeating `if` checks for clearer logic.
* `defaultBlue` is inlined because it is only used once for each OS.
- Updated the `Button.js` file in an existing project and all buttons behave exactly the same. Here is a simple demo:

- There is no change in the underlying logic of the code.
Closes https://github.com/facebook/react-native/pull/14455
Differential Revision: D5330112
Pulled By: hramos
fbshipit-source-id: 7fd1a0609f0bb2123208d0e1b3da2bc779f9583d