Using SDK Version variables from root project

[rayronvictor]

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:
react/react-native#17741
react/react-native#18095

[jaimecbernardo]

Hi @rayronvictor,
Thank you for the contribution.
I was giving it a test, but it seems that the plugin was built with compileSdkVersion and buildToolsVersion greater than the ones react-native is using.

The behavior is as if the ext variables were not defined.
Where in the root project is the plugin supposed to be reading these values from?
Thank you, in advance.

[jaimecbernardo]

A recent released change in React-Native relates to this PR. Newly created project-wise definitions seem to be what this PR's changes refer to.
react/react-native@0a3055d

[staltz]

We're maintaining this module now as a fork at https://github.com/nodejs-mobile/nodejs-mobile-react-native