Trebuchet: Centers vertically the wallpaper

Actually, the wallpaper is not center vertically (both, with wallpaperhack and
without wallpaperhack). This patch modifies:

* Replaces the WallpaperOffsetInterpolator class by the Launcher2 implementation.
* Fixes mWallpaperHack. onDraw now centers the y vertically.
* Disable mWallpaperHack when there is only one homescreen (otherwise a black screen is displayed).

Change-Id: Ia5e0763c9a31609db8f7826f619ed808041e97e7
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge "Trebuchet: Centers vertically the wallpaper" into cm-10.1