Fix shadow painting in layoutlib

Shadows could be painted outside of the allowed drawing region
for the components for which they are the shadow.

Bug: http://b.android.com/215402
Change-Id: I2d2821b745147f3723e8f11d648094fcd684fe51
(cherry picked from commit 9702fffc768db43d0aba4fb1bea54af50af11361)