2° batch of changes

[CampelloManuel]

This time, I

1. fixed the code for notifications
2. edited the build.gradle file
   • disabled proguard in debug mode
   • prepared for desugar
   • made the android tests work again
3. deleted files and strings related to Dropbox™ and billing on the google play store

[CampelloManuel]

@spacecowboy I would like some short feedback on this PR, in case I made some wrong choices.

For the next ones I will do some minor changes that will not require your attention

[CampelloManuel]

the last 3 commits implement your suggestions, so I consider this done.