Add path ops API

Path ops can be used to combine two paths instances in a single path
object. The following operations can be used:

- Difference
- Reverse difference
- Union
- XOR
- Intersection

To use the API:

Path p1 = createCircle();
Path p2 = createRect();

Path result = new Path();
result.op(p1, p2, Path.Op.DIFFERENCE);

This code will subtract the rectangle from the circle and generate
the resulting path in "result."

Change-Id: Ic25244665b6691a7df0b0002a09da73d937b553b