Allow SDK versions to be overridden

[jeaye]

This is a common pattern now, recommended by Google, in the following documentation: https://developer.android.com/studio/build/gradle-tips#configure-project-wide-properties

Many other libraries are taking the same approach, as shown in the following:

• https://github.com/react-native-community/react-native-linear-gradient/pull/232
• https://github.com/react-community/react-native-maps/pull/2047
• https://github.com/transistorsoft/react-native-background-geolocation/blob/master/android/build.gradle#L3
• https://github.com/rebeccahughes/react-native-device-info/blob/master/android/build.gradle#L3

[lucasferreira]

Hi @jeaye thanks for this man ;)