Update Android dependencies and add AndroidX support

[pcsantana]

Motivation

Solves the issue discussed in android-sdk repo, to be able to use newer versions of firebase messaging.

What

Updated the android dependencies versions, including aeroger-cordova-push. Also, I needed to migrate the project to AndroidX. To make it easy, I updated the README with a new topic with instructions to migrate to AndroidX.

Why

Without that, I could not use newer versions of firebase, like 19.0.0.

Verification Steps

1. Remove the android platform
2. Remove the current plugin
3. Install the plugin from this fork with setting a new firebase version (e.g. --variable FIREBASE_VERSION=19.0.0)
4. Install the 2 plugins to migrate to AndroidX (as described in the README):
   cordova plugin add cordova-plugin-androidx
   cordova plugin add cordova-plugin-androidx-adapter
5. Add the latest android platform and run

Checklist:

• Code has been tested locally by PR requester
• Changes have been successfully verified by another team member

Progress

• Finished task

Additional Notes

PS.: I could not use it without to migrate to AndroidX. Maybe there is another way, but this is working for my purpose.