Removing ContentObserver in launcher

> Using callback insted of using a content observer
> Setting the listener in LauncherAppState to prevent resource leak

Change-Id: Id23a4d5c8812e86178997e536226e09ec3740f84
diff --git a/src/com/android/launcher3/WidgetPreviewLoader.java b/src/com/android/launcher3/WidgetPreviewLoader.java
index 75952d1..c1cc42b 100644
--- a/src/com/android/launcher3/WidgetPreviewLoader.java
+++ b/src/com/android/launcher3/WidgetPreviewLoader.java
@@ -27,6 +27,7 @@
 import android.os.Handler;
 import android.util.Log;
 import android.util.LongSparseArray;
+
 import com.android.launcher3.compat.AppWidgetManagerCompat;
 import com.android.launcher3.compat.UserHandleCompat;
 import com.android.launcher3.compat.UserManagerCompat;
@@ -69,7 +70,7 @@
     private final MainThreadExecutor mMainThreadExecutor = new MainThreadExecutor();
     @Thunk final Handler mWorkerHandler;
 
-    public WidgetPreviewLoader(Context context, InvariantDeviceProfile inv, IconCache iconCache) {
+    public WidgetPreviewLoader(Context context, IconCache iconCache) {
         mContext = context;
         mIconCache = iconCache;
         mManager = AppWidgetManagerCompat.getInstance(context);