Migrate to gradle declarative plugin

[hjiangsu]

Pull Request Description

This PR updates our Android project to use gradle's declarative plugin. To read more about the migration, see here: https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply.

@micahmo if you could attempt to build this and run it on your device, that would be great! More specifically, I would check to see if local notifications work after this as background_fetch and flutter_local_notifications require setup that changes these files.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[micahmo]

LGTM!

I built and ran this branch on my physical device, and it seems to be working well! I tested local notifications via the debug page (both the immediate and delayed/background notifications) and they worked fine!