systemui: Fix race condition in setting notification panel height

 * setExpandedHeightInternal has multiple entry points, including from
   an animator. A very subtle race condition exists which was causing
   the blur effect to flicker. So subtle in fact that a heisenbug was
   created when logging to debug the issue.
 * Make the damn thing synchronized so this can't happen for good.

Change-Id: I14a4cd3824a5665e4c7198fb7f61bf0ca9ab2b79
(cherry picked from commit 257b3d48)