Using SDK Version variables from root project

Instead of assuming the `compileSdkVersion`, `targetSdkVersion`, etc, read it from the root project.
Default `compileSdkVersion` and `targetSdkVersion` to the latest versions.

Android Target API Level 26 will be required in August 2018.
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html
And the React Native team is already working on this:
facebook/react-native#17741
facebook/react-native#18095