samples: mesh: nrf52: revised status resp. behaviour & TT calculation

With this it is possible to get proper target value & remaining
time in status response from Servers when transition in progress.
If transition is not in progress then those things would not
get integrate into status response. This is as per Bluetooth
Mesh Model specification.

It is inspired concept from Bluetooth Mesh Developer study
Guide.

Also updated Transition Time (TT) related parameter calculation.

Signed-off-by: Vikrant More <vikrant8051@gmail.com>