code: public boolean initSession(boolean isReferrable, @NonNull Activity activity) {
          return initSession((BranchReferralInitListener) null, isReferrable, activity);
      }
docs: <p>Initialises a session with the Branch API, specifying whether the initialisation can count
  as a referrable action, and supplying the calling {@link Activity} for context.</p>
  
  @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.