Consider open sourcing our wrappers for onbackpresseddispatcher

[rjrjr]

View.backPressedHandler and BackButtonScreen rely on everything being called in the right order across the entire view tree on every update. This is a fragile scheme.

@helios175 has been working on something in house that improves our ability to nest AndroidX back handlers, which would obviate this concern entirely. We should open source it. It fixes this problem, and is more battle tested than View.backPressedHandler.

[rjrjr]

We've learned OnBackPressedDispatcher better, and in particular its integration with Lifecycle, and now think it's good enough.

[rjrjr]

Closed this too soon, still murky waters. Why oh why can't we nest OnBackPressedDispatcher!?