Fix sw.js overwrite

[luisjoserivera]

• Workbox options should be under pwa.
• Both scripts (OneSignalSDKWorker.js and OneSignalSDKUpdaterWorker.js) should be modified to import sw.js in accordance with OneSignal documentation https://documentation.onesignal.com/docs/onesignal-service-worker-faq#integrating-multiple-service-workers

Fixes #1

[codecov]

Codecov Report

Merging #19 into master will decrease coverage by 5.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   75.00%   70.00%   -5.00%     
==========================================
  Files           2        2              
  Lines          48       50       +2     
  Branches        9       11       +2     
==========================================
- Hits           36       35       -1     
- Misses         12       15       +3     

Impacted Files	Coverage Δ
lib/module.js	64.10% <66.66%> (-6.17%)	⬇️

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