Fixing crash on pre-api level 19 devices due to getAlpha() call on Drawable.

Bug: 24173665
Change-Id: Ia59481dc7fb52454760c688ba49c90b8507d7900

src/com/android/launcher3/allapps/AllAppsBackgroundDrawable.java

@@ -35,6 +35,7 @@ class TransformedImageDrawable {
     private float mXPercent;
     private float mYPercent;
     private int mGravity;
+    private int mAlpha;
 
     /**
      * @param gravity If one of the Gravity center values, the x and y offset will take the width
@@ -50,10 +51,11 @@ class TransformedImageDrawable {
 
     public void setAlpha(int alpha) {
         mImage.setAlpha(alpha);
+        mAlpha = alpha;
     }
 
     public int getAlpha() {
-        return mImage.getAlpha();
+        return mAlpha;
     }
 
     public void updateBounds(Rect bounds) {