Restore Compat class for Nook Simple Touch

See discussion on the [mailing list](https://groups.google.com/forum/#!msg/anki-android/OSPCS_UjTVw/n3I24mpTSOEJ). A blank `setScrollbarFadingEnabled` was the only difference between `CompatV4` and `CompatV7`. Adding a dedicated `CompatV7NookSimpleTouch` class should better document the shortcoming of the device.