Add AndroidX to 'Flutter for Android developers' page

[neiljaywarner]

Page URL: https://docs.flutter.dev/get-started/flutter-for/android-devs Page source: https://github.com/flutter/website/tree/master/src/docs/get-started/flutter-for/android-devs.md

Please add an entry about AndroidX

[atsansone]

@mossmana : If you have any pointers for this change, they would be appreciated.

[mossmana]

I've added this to our triage.

[mossmana]

@atsansone There appears to already by documentation around AndroidX at https://docs.flutter.dev/release/breaking-changes/androidx-migration, although it is in the Breaking Changes section which may be the wrong spot for it at this point. Perhaps it makes sense to move it to a more appropriate place on the website.

[mariamhas]

assigning to myself to go through our website and docs and see where else it might make sense to include details around AndroidX migration, libraries etc

[atsansone]

@mariamhas : This was last updated last year. Any thoughts on getting back to this?

[mariamhas]

moving to backlog, need to do some investigation on our support before I can write these docs