LayoutLib: replace the stack of Graphics2D with custom snapshots.

The first step is to allow the canvas to save only the matrix or
the clip. Previously, since the graphics2D were created completely
new on each save, everything was saved.

Also allows to set transform/clip on the Canvas when no bitmap
has been set (and therefore no Graphics2D exists).

This is required by BitmapFactory.

Change-Id: I37eff6dde1ca708d58ba317b40e9b3236f3ef3d0