libui/libgui: Fix errors in parceling

BufferItem and GraphicBuffer were not parceling correctly, which had
not been noticed because the libgui tests (specifically, one that
tests placing a BufferQueue in a separate process from the IGBP/C)
had not been run recently. This change fixes the errors found in
those classes.

Change-Id: Ie224361a534a79115a3481d83ff97f21d154d4f5