SDK 2302 update singleton creation to dispatch once

[echo-branch]

Reference

SDK-2302

Summary

Update to modern singleton creation syntax

Motivation

Code modernization

Type Of Change

Testing Instructions

There should be no functional change

cc @BranchMetrics/saas-sdk-devs for visibility.

[codecov]

Codecov Report

Attention: Patch coverage is 90.38462% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 51.10%. Comparing base (4842da0) to head (1836823).
Report is 6 commits behind head on master.

Files	Patch %	Lines
Sources/BranchSDK/BNCKeyChain.m	66.66%	2 Missing and 1 partial ⚠️
Sources/BranchSDK/BNCEventUtils.m	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1388      +/-   ##
==========================================
+ Coverage   51.03%   51.10%   +0.06%     
==========================================
  Files          66       66              
  Lines       10158    10158              
  Branches     3724     3724              
==========================================
+ Hits         5184     5191       +7     
+ Misses       4716     4707       -9     
- Partials      258      260       +2     

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