Scale windowshade gesture parameters for screen density.

This has been broken since 2.0 but we didn't really notice
until now. Flinging the windowshade should now feel less
janky (the window's motion once flung will be more
continuous with your finger motion while flinging).

Bug: 5210198
Change-Id: Ieae0cabf5def46004f98a6dcb2337b2648f5e9ab