Trebuchet: always set initial wallpaper offsets

After a fresh boot, computeScrollOffset() always returns false. So the
result was the offsets were never properly being set on a fresh boot and
they were only being applied on first drag of the home screen, which
finally triggered computeScrollOffset() to return true.

Change-Id: Iccaedd742bed861fca1f50dd54e2e4537fd4c9b0