Make Auto-scrolling base purely on edge offset, no longer on time.

And lots of tweaks to make everything "better":
-Changed BandController to trigger when near the edge of the app
rather than the edge of the screen.
- Max speed is set to some constant (=70 pixels/scroll) as opposed to
  the view height
- Top/Bottom hotspot no longer using some dp, but rather a ratio of the
  entire view height (1/8 = 12.5%)

Bug: 31743809
Change-Id: Id29f12b0e74b69ef9d632326f7810c2529e204ac