Bug 965367 - Pre: Add AndroidFxAccount.getSyncPrefs.
Bug 965367 - Part 1: Update TestFreshStart for token server Sync.
Bug 965367 - Part 2: Support userSelectedEngines when generating fresh meta/global.
Bug 965367 - Part 3: Pop up dialog when user checks "Choose what to sync".
Bug 965367 - Part 4: Write user's datatype elections to shared prefs.