samples: st_ble_sensor: clean up

This sample is a bit messy; clean it up.

Remove unused includes, add missing include guards, clean up the
separation of responsibilities between main.c and the button and LED
helper files by moving code around, and replace the now-outmoded
device_get_binding() calls with GPIO_DT_SPEC_GET().

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>