A better looking and faster spot shadow.

1. This improve the looking, the star shape (spike) on long action bar is gone.
Shadow is more smooth now.

2. The performance is better, too. For averaging around rect, round rect and
circle, the spot shadow itself is 3 times faster. On N7 v1, it could be less than
0.1 ms.

b/14976551
b/16712006

Change-Id: I61ed546ee56e7c8dbe504dfcaef12d084904b4b8