Optimize handling of scrolled wallpapers

Swiping the home screen causes the WindowManagerService to do
a bunch of work to keep the wallpapers in sync. First, it lays out
and places all windows. Also, it notifies the SystemUI process that
the wallpaper position has changed.

The layout/place operation is too much work - we only need to set
the position values for the wallpaper, not relayout the whole system.

The notification mechanism must exist, but should be optional. Most
wallpapers don't care (especially static ImageWallpapers). So we'll
give them a new API (WallpaperService.Engine.setWantsOffsets()) to
allow wallpapers to opt out of this process and avoid the performance
overhead.

Change-Id: I66c38375438937f14f6f5550565b28eb204b1e06