x/mobile/exp/sprite: pass config.Event to arrangers

[hyangah]

As shown in https://go-review.googlesource.com/#/c/12339/ arranger functions may need the screen dimension info. The CL work around the problem using a global var, but a better approach is to let the sprite engine provides the info it already has.

@nigeltao @crawshaw

[nigeltao]

Should a paint.Event contain a config.Event? Should we do the same for touch.Event?

(Just thinking out loud...)

[nigeltao]

(I realize that that's sort of tangential to the sprite's arrangers.)