ion: Switch ion to use dma-buf

Ion now uses dma-buf file descriptors to share
buffers with userspace.  Ion becomes a dma-buf
exporter and any driver that can import dma-bufs
can now import ion file descriptors.

Change-Id: Ia04d6d72fb301dc088eb8db6576822e9260ff332
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>