samples: nrf52: power_mgr: Update sample to use Zephyr PM policy

This commit updates the power management sample in order to use
new features introduced by recent changes in the Zehyr Power
Management subsystem. Now, it shows how to use power management
policies as well as how to implement custom power modes.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>