This functionality of RingAccount exposes methods to "track" a buddy with it's
ringid. The RingAccount keeps a map of BuddyInfo for all buddies to track. The
BuddyInfo struct contains the last time the device has been seen online.
RingAccount exposes the following methods:

* ::trackBuddyPresence which will look for a buddy's DeviceAnnouncement on the
  DHT to deduce the buddy's presence. This will be done each 10 minutes
  (DeviceAnnouncement expiration time).
* ::getTrackedBuddyPresence which returns the relevant content from the map of
  BuddyInfo, i.e. if each of the buddy is online or not.

Change-Id: Ib149585c4835da88b0b0248b4a866770d35afe90