bluetooth: mesh: brg_cfg: store tbl when changed only

Though nvs backend checks whether the data passed to it is identical to
the stored one, this isn't guaranteed by other the settings api. We can
also avoid going into the settings subsystem since we know what was
actually changed.

This commit adds 2 flags that allow to control whether Subnet Bridge
or Bridging Table states were changed and skips storing if the certain
state was not changed.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>