Correct stale background on completely empty gallery

Bug: 7314560
When the last album is delete by long pressing and pressing
the delete icon, and the gallery is left completely empty,
the slotview was not being correctly invalidated.

Change-Id: I2d5ec7ac7fb9604fd74ca8fa4fa6df09d6dec49d
diff --git a/src/com/android/gallery3d/app/AlbumSetPage.java b/src/com/android/gallery3d/app/AlbumSetPage.java
index eaf9145..1719b89 100644
--- a/src/com/android/gallery3d/app/AlbumSetPage.java
+++ b/src/com/android/gallery3d/app/AlbumSetPage.java
@@ -421,6 +421,7 @@
                 } else {
                     mShowedEmptyToastForSelf = true;
                     showEmptyAlbumToast(Toast.LENGTH_LONG);
+                    mSlotView.invalidate();
                     showCameraButton();
                 }
                 return;