mesh: Correct HB sub state updates

mesh: Correct HB sub state updates

If heartbeat subscription is disabled, all fields should be set to zero
but collected data should be preserved. If HB subscription is enabled,
the collected data should be reset (which includes Min Hops = 0x7f).
HB subscription is disabled by setting any of the following fields to
zero: Source, destination or period log.
HB subscription is enabled by setting all the same fields to valid values.

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>