Consider switching to Pigeon for Android

[Levi-Lesches]

There's a lot of Java and Dart code around parsing/serializing/deserializing notifications, details, and options. I'd be interested in trying out pigeon, which would generate all that based on a few Dart files. That should get rid of lots of hand-written code, and prevent the need for hand-writing parsing code for future features.

If this works, it should be relatively trivial to use Pigeon for iOS/MacOS as well.

[Levi-Lesches]

Computer was on the fritz, see #2504