Keep the player always on top

This is a copy of !285 (closed) , so that the gitlab runner build the code.

Sorry for the size of the PR. I didn't expect it to be that large. Initially I just wanted to refactor so that player bottom bar appears on top of the search view. It is a feature I desired for so long. But then I found myself refactoring a large parts to replace the search activity with a fragment and use Jetpack's navigation to navigate through theses fragments. I also ended up refactoring the search view to use ViewModels as results were not persisted throughout navigation after using a fragment.

I'm really sorry, I tried my best to keep this PR as atomic as possible but it didn't make much sense to me using a fragment without using navigation and viewmodels as this is now the standard pushed by Google. There's still room for improvements here. For instance, the search fragment does not use DiffUtil to refresh the RecyclerView and I think this could be a slight performance improvement.

Closes #107 (closed)