Use correct alpha value when a color is not set on a GradientDrawable.

A previous fix ensured that color filters would have a default (black) color to
interact with if no color was set on the GradientDrawable object. However, that fix
assumed an opaque alpha value, which is not always the case. Specifically, calling
setImageAlpha() on an ImageView with a shape drawable source (as in the bug here)
caused the alpha to be set to a translucent value, which was then ignored in the fix
above.

The fix is to account for the current alpha value of the GradientDrawable object
when setting the color used by the paint object.

Issue #7592193 ImageView.setImageAlpha() broken when colorFilter is in use

Change-Id: Ie622ffca776fdd8731ced78ce1f683ca6a51dec8