LayoutLib: fix clipping issues.

There were two issues:
- Graphics2D.setClip only works on rectangular shapes.
  This means doing a setClip on a non rectangular shape should
  basically reset the clip and intersect with the new shape.

- the current clip can be null, so the combineShape method
  must handle it.

Change-Id: Id2cd7475e991d8b533ff2e8850cc2c27663f9e52