Make the app runnable with Android 6 devices again

[timobaehr]

• Added permission for running the service as ForegroundService
• Added location permission when starting the app
• Added gradle files to build the application with latest Android build tools
• Updated homepage inside README.md (homepage looks like something else)

Open issues:

• no error handling if user does not accept the location permission
• notification is not updated
• If the OS setting is dark, the list of applications is nearly invisible
• On modern devices only two applications are shown: PowerTutor and System

Successfully tested on:

• Sony Xperia Z3 Compact Tablet (Android 6)

Running with errors:

• Huawei P30 (Android 10)
• Pixel 2 (Android 11)