Support for androidx

[AndroidX](https://developer.android.com/jetpack/androidx)  is a new set of libraries that replaces the old Android "supports" libraries, and more.

We don't use supports libraries directly (if i recall correctly), but to activate `androidx` package, it requires a new file named `gradle.properties` with:

```
android.useAndroidX=true
android.enableJetifier=true
```