Fix applying user settings at login

This fixes applying KGamma's user settings at login.
It changes two things:
- export kcminit_kgamma() again, this is necessary so that kcminit can
  call that function to apply the settings
- add "X-KDE-Init-Phase=0" to the .desktop file, which is apparently
  needed since Plasma 5.3 to make it being called at login
CCBUG: 218668
REVIEW: 124171