Add predictive back support

Not so long ago Android introduced a stable API for predictive back gesture that adds contextual transitions between previously opened screens and appa.

https://developer.android.com/design/ui/mobile/guides/patterns/predictive-back

It would be really nice to have such animations used instead of the default back handler that simply swipes down any app.

Here are examples of back gesture from Niagara Launcher and the default Pixel launcher.