Update the VD and AVD's getOpacity()

static VectorDrawable should be returning Transparent while root alpha is 0.
AnimatedVectorDrawable running on RenderThread now, and the opacity should
be conservatively set as Translucent.

http://b/27554766

Change-Id: I9a78696cab94f10ba40a733cc9c00f1c2d7ebb7f