Refactoring getPreferenceKey method

This method was returnning a constant and getting inlined by proguard.

Change-Id: I87348e25b21483adc1b27d16f99dec4b73205701
diff --git a/src/com/android/launcher3/LauncherAppState.java b/src/com/android/launcher3/LauncherAppState.java
index f75b33c..34b8382 100644
--- a/src/com/android/launcher3/LauncherAppState.java
+++ b/src/com/android/launcher3/LauncherAppState.java
@@ -153,14 +153,10 @@
         return sLauncherProvider.get();
     }
 
-    public static String getSharedPreferencesKey() {
-        return LauncherFiles.SHARED_PREFERENCES_KEY;
-    }
-
     public WidgetPreviewLoader getWidgetCache() {
         return mWidgetCache;
     }
-    
+
     public void onWallpaperChanged() {
         mWallpaperChangedSinceLastCheck = true;
     }