Add strings for authentication type.

A new field is going to be added to the APN editor.
Authentication type: 4 possible values - None, PAP, CHAP, PAP or CHAP
These strings are too technical for the end user, however these
fields will be pre-populated and the user would not have to change them.

Change-Id: I7df326722fc4aebfe68d2726e2695eda8ea798a9