Fix live wallpaper stays on AOD mode

After unlock the device, the live wallpaper status stay on AOD mode.

WallpaperConnect does not disconnect while users existing wallpaper
app by entering home or pressing back key.

Disconnect WallpaperConnection in onStop().

Video:
https://drive.google.com/file/d/13Ek4ZSyjNHeUcAv-nh9uT8daYHN4i7rm/view

Bug: 158434793
Test: Manual
Change-Id: I0eac6ff1bf6ba9be9d51987a330c1b1bd9c2bcd4
diff --git a/src/com/android/customization/picker/WallpaperPreviewer.java b/src/com/android/customization/picker/WallpaperPreviewer.java
index c8b4bf2..03e7031 100644
--- a/src/com/android/customization/picker/WallpaperPreviewer.java
+++ b/src/com/android/customization/picker/WallpaperPreviewer.java
@@ -89,6 +89,15 @@
         }
     }
 
+    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
+    @MainThread
+    public void onStop() {
+        if (mWallpaperConnection != null) {
+            mWallpaperConnection.disconnect();
+            mWallpaperConnection = null;
+        }
+    }
+
     @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
     @MainThread
     public void onDestroy() {