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: I8851f5fbdebfeeef9459c5b44c599e489b2dda3a