afb-session: Use ad-hoc pseudo-random uuid

The use of the generator given by the lib uuid
implies the use of the system call 'getrandom'
without the flag GRND_NONBLOCK. It has a weird
effect on boot because many binders require
the generation of uuid at the same time but
getrandom is subject to starving.

This introduced a tiny pseudo-random generation
of UUID that does not implies the use of 'getrandom'.

Bug-AGL: SPEC-1655

Change-Id: I5131072881d7a53f0edda9e36762985c96a04550
Signed-off-by: Jose Bollo <jose.bollo@iot.bzh>