Clean contentView_ reference in onDestroyView()

If setRetainInstance(true) has been set on the fragment or
addToBackStack() was applied to FragmentTransaction, the fragment won't be
destroyed and will keep the reference to its parent view in contentView_
and could lead to memory leak.

This change implements onDestroyView() which cleans the reference.
However, the developer is still responsible for cleaning up the injected
views, because otherwise a (badly implemented) background task may get a
NullPointerException.