read project vars for compileSdkVersion, et al

[abonander]

Following this convention helps prevent headache-inducing build errors from version mismatches.

[erezrokah]

Hey @abonander and thanks for the pr :)

Do you mind describing (maybe opening) the issue that this pr fixes (an example repo will be perfect)?
It is hard for me to understand the changes this way.

Also will this change the existing defaults for the project values?

[abonander]

I put the defaults back, that was a mistake.

This is important when a project wants to specify different versions for these parameters, as having dependencies with differing versions can cause build errors. It is especially important to have compileSdkVersion match the version of any support libraries that are being linked. The user will define these variables in their top-level build.gradle: https://developer.android.com/studio/build/gradle-tips#configure-project-wide-properties

This is a convention that other packages are already following (the code is equivalent):

• https://github.com/zo0r/react-native-push-notification/blob/master/android/build.gradle#L26
• https://github.com/transistorsoft/react-native-background-geolocation/blob/master/android/build.gradle#L20
• https://github.com/rebeccahughes/react-native-device-info/blob/master/android/build.gradle