just added a function which acts like a ternary with fallback option. Hence, less cluttered "def" variables  
additionally, changed the SDK values from 23 to 26 as per new changes from react-native and Android

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#18095
facebook/react-native#17741

PS: I am aware of this PR https://github.com/joltup/rn-fetch-blob/pull/128 but first its still targeting old SDK values i.e 23 as fallback and secondly I am not sure of the use of `project` to get the value instead of proper way i.e `rootProject.ext` to get the property value.