Incorporate alpha attribute when painting VectorDrawable

When painting a vector drawable, alpha transparency can come from
the path color, or from the overall android:alpha attribute.
Include both when drawing.

Bug: http://b.android.com/206667
Change-Id: Id946fdeaf72b981597787f5357ef3a90a471c584
(cherry picked from commit 8cae3b0ce6f527e7b90d46755ffaf6d3d4d65114)