Support drag-to-dismiss in persistent bottom sheets

Flinging a persistent bottom sheet downwards dismisses it, per the Material spec.

The showBottomSheet() function now returns a Future, like showModalBottomSheet() does, so that you can discover when it's been dismissed - with navigator.pop() - and with what value.

Factored the drag gesture handling code into _BottomSheetDragController

This CL was flutter/engine#1997