Wallpapers, animations, pending intent.

Some more tweaks and fixes to wallpapers.  Make sure wallpapers are
told they are not visible when the screen is off.  Add some new animations
for transitions across tasks, and fiddle with many of the existing
animations.  Clean up the relationship between translucent activities
and animations.  Add new API to start a PendingIntent from an
activity.

Change-Id: Ie0bf45fe44081bb6982c75361257a55d9cd9d863