Refresh pref settings for cloud sync

- Show the opt-in text
- Show the account picker if user presses 'enable sync'. Previously
  we disabled the sync pref which was confusing.
- Remove the debug tags from some prefs; and improve messaging overall

General rules;
- sync is turned ON : user checks 'enable sync' pref AND
  accepts the opt-in AND chooses an account
- sync is turned OFF: when user signs out

Demo link
https://drive.google.com/a/google.com/file/d/0B9tNQOWdRuiWSUdVVE5rVDJudlk/view?usp=sharing

Change-Id: I2e7933796b15e47005ba9970a8c1294416ef31a0