Lowered gradle versions to react defaults
Lowered gradle versions to react defaults

* Added compileSdkVersion and buildToolsVersion ext override settings to future proof
* Locked the OneSignal native Java library to an exact version
* Misc clean and comments to the build.gradle

geektimecoil/react-native-onesignal,geektimecoil/react-native-onesignal,geektimecoil/react-native-onesignal