Bug 1018534 - [email] switch manifest type to privileged

Tested on Flame via a reset-gaia with latest gaia master and a nightly mozcentral gecko flash:

* Tested contacts interaction: was correctly not prompted for Contacts permission since installed on phone as part of build, fetching contact for display in message list/reader was correct, could still compose and add from contacts.

* Tested l10n switching: the email app did get locale changes. It still had some issues, tracked already in bug 1005760, but unrelated to the privileged switch -- for already in DOM elements, the locale was updated.

* account setup, mail fetching and sending still work. Sound still plays when sending an email and that in-app pref is turned on.