update build gradle version

Kindly update the Gradle version. Everything was functioning correctly until a few days ago when I stopped receiving notification tokens. After updating the Gradle version from 3.2.0 to 3.2.1, the issue was resolved seamlessly.

react-native-push-notification/android/build.gradle

dependencies {
    classpath 'com.android.tools.build:gradle:3.2.0'
}