Fix issue #8540550: Screenshots should never have transparency

Make sure the thumbnail bitmap is initialized with a black
background.

Change-Id: I87c00d309b7d30b6085a751aa4ae22cf146c74f5

services/java/com/android/server/wm/WindowManagerService.java

@@ -5490,6 +5490,7 @@ public class WindowManagerService extends IWindowManager.Stub
         ScreenRotationAnimation.createRotationMatrix(rot, dw, dh, matrix);
         matrix.postTranslate(-FloatMath.ceil(frame.left*scale), -FloatMath.ceil(frame.top*scale));
         Canvas canvas = new Canvas(bm);
+        canvas.drawColor(0xFF000000);
         canvas.drawBitmap(rawss, matrix, null);
         canvas.setBitmap(null);