Trebuchet: Cleanup preferences

* Remove misleading summary from Homescreen settings. It says "Screens and wallpaper"
but there are no wallpaper settings inside. It also seems to be unnecessary,
removing it brings us inline with the rest of the preferences
* Remove unused string: preferences_interface_title
* Hide Drawer & Dock settings until we actually have some settings there.
Users freak out when they tap these preferences and nothing happens.

Change-Id: Iedd4c809a0a8fbf17e22a65b941fcd43e662da3f