tests: subsys: settings: Use DT_FLASH_AREA_<FOO>_ID define

Set Kconfig symbol SETTINGS_FCB_FLASH_AREA based on the
DT_FLASH_AREA_<FOO>_ID define instead of being hard coded.

We replace 3 with DT_FLASH_AREA_IMAGE_SCRATCH_ID and
we replace 4 with DT_FLASH_AREA_STORAGE_ID.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>