FlutterFragment predictive back

[justinmc]

Following up on https://github.com/flutter/engine/pull/39208 and https://github.com/flutter/engine/pull/42789, which added predictive back support for FlutterActivity.

My test app for this is: https://github.com/justinmc/flutter-add-to-app

I tried this both with the Flutter app as the main activity, and with it as a secondary activity after an initial Android route. Predictive back worked in both cases.

Related: https://github.com/flutter/flutter/issues/109558, and my comment https://github.com/flutter/flutter/issues/109558#issuecomment-1683099477.

b/242216228

[goderbauer]

I spoke to @justinmc last week and he told me that he will get back to this one soon.

[justinmc]

I'll re-evaluate this after https://github.com/flutter/engine/pull/49093 is merged.

[justinmc]

Spiritual successor: https://github.com/flutter/engine/pull/52302