Bug 1037395 - Overriding profile value with partner preferences on common prefs

The preferences from PARTNER_PREF_FILES are appended
to profile/user.js but if there's a common preference
on both user.js and PARTNER_PREF_FILES, user.js preference
is replaced with the partner's preference value.

Change-Id: Ie0beb98d33470d9ae91dd4cbf948f89219d8b94f