Output dir is now cached across app launches

[theGeekyLad]

Fix for issue #11

• Included a new prefs key in KeyCollection.java to store chosen output dir path
• On choosing output dir for the first time round
  • The Uri is stored in prefs against the :point_up: key
  • Since Uris perish once the activity is destroyed (nice read here), takePersistableUriPermission() for this chosen outputDirUri
• Added a new method readOutputDirUriFromCache() that does Uri.parse(...Uri string from prefs...) to get the Uri back (basically "uncaches" it back)
• All outputDirUri null checks have been enhanced with also checking if the "uncached" Uri from :point_up: also is null

[theGeekyLad]

Also @npes87184 , would really appreciate if you could add the topic hacktoberfest to this repository and accept my PR (if you deem it valid) with the label hacktoberfest-accepted

It would count as a contribution for me!

[theGeekyLad]

Hey @npes87184 , I've made the changes that you'd indicated and updated this PR. Kindly check it out when time permits. Thanks! :+1: