Merge pull with updates from API level 21 to API level 28

[MarcoTTC]

Hi. You may not know me, but my name is Marco, and I'm trying to make a break in my career as a mobile software developer. Thus, I've decided to contribute to some open source GitHub mobile projects I've seen around. I've chosen SoundRecorder as my first try, and I've updated the project to build and deploy as a target API level 28 android project, modifying the code whenever needed to make it so. I've tried to make my commits small and didactic as possible. Since I put some effort into making sure the code compiles, I hope you can appreciate my changes to the code, and accept the merge pull request, but if you could also launch a new version on Google Play, that would be great. Also, I hope to add this project to my portfolio as something I've contributed code towards to, respecting the original authorship. Thank you for your time, and I hope you like my efforts with SoundRecorder.

[naXa777]

1. You request permissions at runtime in RecordFragment, but don't do this in PlaybackFragment. Let's assume that permissions for recording (MIC + STORAGE) were granted and then revoked. In this case, when user goes to recordings tab and plays a recording, what will happen? Does the app handle this case properly?
2. Android Pie requires a special permission and a persistent notification for sound recording in background. Please verify this use case as well.

In general, you've done good work! You just need to test the mentioned use cases and apply changes if required.

[MarcoTTC]

Thanks for mentioning the special audio permission and notification for Android Pie. I'll make sure to double check the proper audio requisition and usage in Pie. Also, I may be mistaken, but I think only external data write permission and microphone recording are considered dangerous by Android standards, while audio playback and external data reading are not, but I'll double check it too and implement the proper changes when I have the time. Thank you again.