Don't reset Enigmail settings on Thunderbird restart (refs: #17447)

Using "pref" in the AutoConfig file resets the value every time
Thunderbird is restarted. If we want the user to be able to change the
value without it being reset, we should use "defaultPref" instead.

FTR, the various functions of AutoConfig are described here:

https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig#w_functions-of-autoconfig