Cut ripple background alpha in half

Ripple alpha is supposed to be split evenly between the foreground
ripple layer and the background layer, but the background alpha wasn't
getting adjusted properly.

BUG: 17658817
Change-Id: I7af2f2ed38400a40d4a17da020363c7ae5c71a7b

graphics/java/android/graphics/drawable/RippleBackground.java

@@ -106,7 +106,7 @@ class RippleBackground {
 
     public void setup(int maxRadius, int color, float density) {
         mColorOpaque = color | 0xFF000000;
-        mColorAlpha = Color.alpha(color);
+        mColorAlpha = Color.alpha(color) / 2;
 
         if (maxRadius != RippleDrawable.RADIUS_AUTO) {
             mHasMaxRadius = true;