API REVIEW: VectorDrawable

- Merge <size> and <viewport> attributes all in to top-level <vector> tag
- Indent attributes under <group> in java doc.
- Updata android:stroke to be android:strokeColor, likewise android:fill
- Instead of android:clipToPath, make this a different clip-path tag.
- Document units of the various attributes
- Add example code for defining a VectorDrawable resource

More than that:
= Refactor the code to better support clipPath as a sub-class.
= Update all the xml files to use the new attributes and clip-path tag.

TODO:
-- Remove clipToPath, since that should happen on build break Friday.

bug:16488254

Change-Id: I6db5680ef83cb26c8f064a60fc7d6e7142974b0f