Bug 966103 - Part 1: Expose last synced times to Firefox Accounts.

There are a number of places this could go; I like this one because it's
easily available to consumers while being forward looking enough that I
can see us adding additional timestamps (in different prefs collections)
for additional services.  Other places include manually managed prefs
and hanging off the clients data delegate.
Bug 966103 - Part 2: Add a 'Sync now' button to the main FxA status screen.
No bug - Roll back overzealous rewrites. r=trivial

This code ships and is used; we can't avoid it.  We could override the
v9 API (apply) but we can't override the v10 API ({get,put}StringSet)
so let's not bother with either.
Bug 966103 - Add a 'Sync now' button to the main FxA status screen. r=nalexander,rnewman