Settings: Generic Blacklist support

Change-Id: Ib5e0d69834506e9fb80f61136b34b26638c72967
Fix broken target icon.

Steps to reproduce:
- Assign an icon from file
- Save
- Reopen lockscreen targets
- Assign a resource icon
- Save

Result: You'll get the application icon, because the icon file reference
is still present, but the file is no longer there.

Change-Id: I6212c59ad3c792b15c7368436b81c6a866db0726