samples: nrf52: power_mgr: Increase idleness for main thread

Increase sleep time for main thread (1 sec to 1 minute) to keep SOC
in Low Power State for longer time. Currently nrf SOC is entering into
low power state and exiting immedately after 1 sec.
With this change SOC will wake stay in Low Power State till GPIO is
pressed or sleep time expired (whichever is earlier).

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>