Nuke WindowState#mShownPosition. Rework mXOffset/mYOffset.

Ignoring Wallpaper Offsets, the WindowStateAnimator is now
always positioned at (0,0), so we don't need to calculate or store this. For
Wallpaper Offsets we can manipulate the position of the WindowStateAnimator surface
directly. This seems to be a nice level to model the concept of scrolling a buffer
larger than the "Window" to which it is assigned.
Everything on top of WSA can ignore the offsets by only interacting with the WS and above.

Test: Manual. go/wm-smoke
Change-Id: I631ad97bb07b092e795c9530ca34139ccc3e0af7