Sticky prefs are already specifiable with `sticky_pref`, but this is a more
general attribute mechanism. The ability to specify a locked pref in the data
file is new.
The patch also adds nsIPrefService.readDefaultPrefsFromFile, to match the
existing nsIPrefService.readUserPrefsFromFile method, and converts a number of
the existing tests to use it.
MozReview-Commit-ID: 9LLMBJVZfg7