Rename Skin::getUsableSkins() to Skin::getAllowedSkins()

Follow-up to change 65426 (commit a3c7a8b8f)

Also, rewrite the documentation for this function call in Preferences.php
To remove the "(skins is all skin names from Language.php)"
which I couldn't make sense of.

Change-Id: I0b0fdfb5615c22aa876882c3ca344ebb1f10c4bd
Merge "Rename Skin::getUsableSkins() to Skin::getAllowedSkins()"