Add support for swipe with multiple fingers

This CL replaces the usage of Modifier.draggable by a custom
Modifier.pointerInput that returns the number of pointers down to
DraggableHandler.onDragStarted when a drag starts.

The current logic is pretty simple and mostly forked from Draggable.kt
and DragGestureDetector.kt: we still wait for the *first* pointer down
to reach the touch slop, then we count the number of distinct pointers
that are down/pressed. In the future, we might want to make the
implementation more complex and wait for any pointer down to reach the
touch slop, instead of just the first one.

Bug: 291055080
Test: SwipeToSceneTest
Change-Id: Ifdb3f118b6765a8b6d07b3b4fe0edd78273b33c2