Make Flattenable not virtual (libutils)

Making an object Flattenable doesn't force it to
become virtual anymore. For instance, Fence and GraphicBuffer
are now non-virtual classes.

Also change Flatennable protocol a bit so that it updates
its parameters (pointers, sizes) to make it easier
to implement a flattenable in terms of other flattenables.

Change-Id: Ie81dc7637180b3c2cfcbaf644f8987ca804eb891