Add Settings.WallpaperSettingsActivity

Provide a way to open wallpaper settings directly.

Bug: 26688904
Change-Id: I3e862b45ad8a4cec59f89495c993f0852c720e55

src/com/android/settings/Settings.java

@@ -134,6 +134,7 @@ public class Settings extends SettingsActivity {
         /* empty */
     }
     public static class ScreenLockSuggestionActivity extends ChooseLockGeneric { /* empty */ }
+    public static class WallpaperSettingsActivity extends SettingsActivity { /* empty */ }
 
     // Categories.
     public static class WirelessSettings extends SettingsActivity { /* empty */ }