libgui: Add a union to BufferItem for refactoring

Currently, there are two instances of BufferItem: one inside of
IGraphicBufferConsumer, and a standalone one inside of libgui. They
only differ in the name of one of the fields, so this change modifies
the one inside of libgui to have a union of both names so that the
one inside of IGBC can eventually be refactored away.

Change-Id: I64f495105f56cbf5803cea4aa6b072ea29b70cf5