IOMX: add updateGraphicBufferInMeta method for metadata mode

This is used to set the handle in the metadata buffer that is
valid in the mediaserver process, as well as to keep a reference
for the graphic buffers in BufferMeta.

Change-Id: I46bb68d8bed894f357eeeb25368360d11b276634
Signed-off-by: Lajos Molnar <lajos@google.com>