Internal: b/242216228

Per https://developer.android.com/guide/navigation/predictive-back-gesture onBackPressed is being deprecated, and we need to switch to OnBackPressedDispatcher and OnBackPressedCallback.

There's been some previous discussion related to this in #67011. See also #109513 for the overall predictive back.

I'm filing this separately to track the initial step of just migrating to the newer APIs, without trying to address larger feature questions discussion in those issues.