code: public boolean initSession(BranchUniversalReferralInitListener callback, boolean isReferrable, Activity activity) {
          initUserSessionInternal(callback, activity, isReferrable);
          return true;
      }
docs: <p>Initialises a session with the Branch API.</p>
  
  @param callback     A {@link BranchUniversalReferralInitListener} instance that will be called
  following successful (or unsuccessful) initialisation of the session
  with the Branch API.
  @param isReferrable A {@link Boolean} value indicating whether this initialisation
  session should be considered as potentially referrable or not.
  By default, a user is only referrable if initSession results in a
  fresh install. Overriding this gives you control of who is referrable.
  @param activity     The calling {@link Activity} for context.
  @return A {@link Boolean} value that returns <i>false</i> if unsuccessful.