Migrate web_dashboard to v2 Android embedding

[craiglabenz]

$ flutter pub get
Running "flutter pub get" in web_dashboard...                    2,546ms

This app is using a deprecated version of the Android embedding.
To avoid unexpected runtime failures, or future build failures, try to migrate
this app to the V2 embedding.

Take a look at the docs for migrating an app:
https://github.com/flutter/flutter/wiki/Upgrading-pre-1.12-Android-projects

The plugin `cloud_firestore` requires your app to be migrated to the Android
embedding v2. Follow the steps on
https://flutter.dev/go/android-project-migration and re-run this command.

Alternatively, it's possible that this web demo doesn't even need an android/ folder.

[craiglabenz]

cc @johnpryan RE the viability of deleting web_dashboard/android/ entirely

[johnpryan]

Yep, we can delete it