Significant update to session management, data handling and RSSI

* Session management fully encapsulated in new "SessionManager" class.
* HeartRateMonitorActivity now responsible for UI functionality only.
* HeartRateMonitorService now responsible for instantiating SessionManager, SessionData and ConnectionManager, and ensuring persistence throughout session.
* SessionManager configured by dependency injection.
* RSSI now available (and displayed); ANT+ extended data is enabled via custom antLibConfig() function.
* MonitorSession class renamed to "SessionData".
* Application persistence improved.

Signed-off-by: Adam Thwaites <thwaites.adam@gmail.com>