Push notifications android documentation update

[AbirAbbas]

The step 7 in the following link is outdated!

https://docs.amplify.aws/lib/push-notifications/getting-started/q/platform/js#setup-for-android

Instead of

dependencies {
compile project(':@aws-amplify/pushnotification')
..
..
..
compile 'com.google.firebase:firebase-core:12.0.1'
compile 'com.google.firebase:firebase-messaging:12.0.1'
}

I have used the following:

dependencies {
...
...
implementation 'com.google.firebase:firebase-analytics:17.2.2'
implementation 'com.google.firebase:firebase-core:17.2.0'
implementation 'com.google.firebase:firebase-messaging:20.1.0'
}