net: lwm2m: relocate/memory align notification_attrs struct

For code clarity, let's move notification_attrs structure to the top
of lwm2m_engine.c.  While we're at it, we can re-order it's members
for memory alignment.

NOTE: This patch does not change the current flash or SRAM usage but
further additions to the notification_attrs structure could.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>