Add new wallpaper features for insets and offsets.

Add new wallpaper features for insets and offsets.

Issue #17394151: WallpaperService / Engines need to get notified
of WindowInsets

Issue #17394203 Wallpapers need a system API to be shifted in order
to support burn in protection

Adds a new API on WallpaperManager to set additional offsets to
make wallpapers extend beyond the display size.

Insets are now reported to wallpapers, to use as they may.  This
includes information about the above offsets, so they can place
their content within the visible area.  And to help with this, also
expose the stable offsets APIs in WindowInsets which is also very
useful information for the wallpaper.

Another new API on WallpaperManager to set a raw offset to apply
to the wallpaper window, forcing it to move on the screen regardless
of what the wallpaper is drawing.

Fix wallpapers when used with overscan enabled, so they still extend
out across the entire screen.  Conveniently, the above new window
insets information is very useful for this case as well!

And a new wallpaper test app for all this stuff.

Change-Id: I287ee36581283dd34607609fcd3170d99d120d8e