onAttach/onDetach possible issues for Fragments

While working on #6906, I realized that we don't use the support version of the Fragment which leads to some issues with onAttach and onDetach. I haven't gone through the whole code base just yet, so I don't really know how big of a problem it is, but here is the problem:

onAttach(Activity activity) has been deprecated on API 23 and instead onAttach(Context context) has been introduced. Unfortunately, the context version does not get triggered for "< API 23" which means that code is never executed. The same thing applies for onDetach. I think the correct way to get around this issue and many possible other issues in the future is to use the support version of the Fragment, or any other UI component really. The regular version of Fragment classes never get updated and these issues come up, whereas the support versions are regularly updated so we don't need to deal with them.

As mentioned, I don't know how much this affects right now aside from #6906 just because I don't have the bandwidth to look into it today. However, if we agree this is the right course of action, I can have a look at this next Monday (December 11) when my Groundskeeper rotation starts.

/cc @maxme

Start at the onAttach and onDetach lifecycle entry points and trace Fragment usages related to #6906. Determine how the current platform Fragment behaves below and at API 23, then assess the scope of switching to support Fragments. Done means the affected callbacks work across the relevant API levels without leaving current Fragment usages inconsistent.