Changes

esim.saip.File: Re-compute file_size when changing body

If the API user modifies the size of the body, we need to check if we
need to re-compute the file_size attribute which is later encoded into
the fileDescriptor.  The size obviously must be large enough to fit the
body.  Let's do this implicitly by introducing a setter for File.body

Change-Id: I1a908504b845b7c90f31294faf2a6e988bdd8049