Enable debuggable build type for debugging

[meenbeese]

This PR introduces a debuggable build variant into the project. Now, with the flag debuggable true and applicationIdSuffix set to .debug, devs are able to generate and run a debuggable version of the app. This is especially helpful on real devices because it helps prevent package name conflicts between debug and release versions and allows them to be installed side-by-side. This change will thus provide more effective identification and fixing of issues during the development process.