ListCursorAdapter: Use an LruCache to avoid repeatedly decoding the bitmap data.

This avoids us repeatedly showing and hiding (with a progress circle
in between) images in the list view while downloading new items, caused by
our RecyclerView.Adapter being invalidated (by us) whenever the number of
items changes.
However, this cache is just a workaround. What we really need is
real cursor-based RecyclerView.Adapter, if that is even possible.