Rename some uses of instance_code to skin_code

instance_code is an overloaded term. Sometimes, we really mean skin_code
Factor out Action bar title change behavior

Make title bar behavior more consistent across different activities.

fixes #155 on github
Always show the app name for skinned apps
Merge pull request #161 from steventlamb/bug/155_inconsistent_app_title

Bug/155 inconsistent app title