LayoutLib: support for layers.

Layers require that drawing methods potentially
draw in more than one bitmaps.

To handle this this patch offers the following:
- move all drawing methods to use Drawable
- Drawables are now handled by GcSnapshot since
  its the one handling the layers
- moved Canvas_Delegate.createCustomGraphics to
  GcSnapshot which does not expose the Graphics2D
  objects anymore so its draw() methods are the only
  way to draw.
- handles creating layers in GcSnapshot.save() and
  blitting them in restore()

Other changes:
- Clean up the create/save API in GcSnapshot
- Fixed drawing bitmaps with alpha and other
  composite modes.

Change-Id: I1e230087493d044a10de71f4b6d29083e3f3bf64