Migrate to AndroidX

[yvolk]

As I understand https://developer.android.com/guide/topics/ui/settings.html we should use AndroidX libraries for new development.

@martin-stone Having your Color Picker as an internal project ( see #51 (comment) ) I realized that before starting to write my new Custom preference I must migrate your component first :-)

And I did this. One day - and it works! Please feel free to integrate these changes into your repository, see this commit with full code of Preferences migration to AndroidX: andstatus/todoagenda@27cce2c

@martin-stone Thank you once again for the beautiful Color Picker.

[martin-stone]

You can use this library with AndroidX without any changes by setting android.enableJetifier=true in your gradle.properties. At some point I'll migrate it fully, but right now the library supports apps using AndroidX and as well as those that have not yet migrated.