Handle premultiplication correctly for ColorMatrixColorFilters

bug:17405627

Previously, the input content to the color matrix computation was left
premultiplied. Since the color matrix could reduce the alpha channel,
the alpha was re-multiplied, but this was incomplete, and incorrect.

Instead, apply the color matrix in unpremultiplied space.

Change-Id: I87b8e03d2e228e6ded81f7bbfea952605d7a095c