Add primitive shadows support to LayoutLib

The shadows rendered are not the same as rendered by the framework. The
main purpose of the shadows here is let the user know when shadows will
be cast.

Currently rectangular shadows are available in only two intensities. Any
elevation below 10dp uses one shadow and any elevation over 10 dp uses
the other.

Also, even though the code for arbitrary shapes exist, it doesn't quite
work yet. No shadows are visible when that code is run.

Change-Id: Ie8235ffccf1d3809713f5d8f82afde434817d6b2