Explicitly bind the calculated implicit height to the flickable contentHeight

Items that can change dinamically their size won't play well with the
auto-created flickable of a SvrollView, as in that case the flickable's
contentHeight needs to be explicitly binded.

Do it after we are sure there is a flickable in the ScrollView as we
don't want to trigger an early creation of  the internal flickable as
the
client code might be wanting to assign a custom created flickable or
item view

BUG:477257