code: public boolean initSession(BranchReferralInitListener callback, Activity activity) {
          if (customReferrableSettings_ == CUSTOM_REFERRABLE_SETTINGS.USE_DEFAULT) {
              initUserSessionInternal(callback, activity, true);
          } else {
              boolean isReferrable = customReferrableSettings_ == CUSTOM_REFERRABLE_SETTINGS.REFERRABLE;
              initUserSessionInternal(callback, activity, isReferrable);
          }
          return true;
      }
docs: <p>Initialises a session with the Branch API, passing the {@link Activity} and assigning a
  {@link BranchReferralInitListener} to perform an action upon successful initialisation.</p>
  
  @param callback A {@link BranchReferralInitListener} instance that will be called
  following successful (or unsuccessful) initialisation of the session
  with the Branch API.
  @param activity The calling {@link Activity} for context.
  @return A {@link Boolean} value, indicating <i>false</i> if initialisation is
  unsuccessful.