fix the overlay effect problem on Android-L

Author: pedant

src/cn/pedant/SweetAlert/widget/SweetAlertDialog.java

@@ -299,6 +299,7 @@ public SweetAlertDialog setConfirmClickListener (OnSweetClickListener listener)
     }
 
     protected void onStart() {
+        getWindow().getDecorView().getBackground().setAlpha(255);
         mDialogView.startAnimation(mModalInAnim);
         playAnimation();
     }