staging/android: rename sync_fence to sync_file

sync_file has a more close meaning to what a sync_fence really, a struct
that represent a file that can be used by userspace to get information on
a fence, or wait for it to be signaled.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>