Add support for swipes started from an edge

This CL adds support for swipes started from a specific edge. The
definition of edge is abstracted away into an EdgeDetector class, which
defaults to a fixed edge size of 40dp.

Bug: 291055080
Test: SwipeToSceneTest
Test: FixedSizeEdgeDetectorTest
Change-Id: I608ccefc26804c588e7568057e58606683cca8d7