Avoid radius 0 gradient painting

If the radius is 0, do not try to paint the shadow. In some views, a 0
radius for the outline might be returned causing an exception when
painting the gradient. In those cases, simply disable the shadow.

Bug: http://b.android.com/207664
Change-Id: Idb937083d42ca02bbace843b881f61aa88fe47f2
(cherry picked from commit 5bc40214817fc6bd4ee541763d40522ae94f1d82)