storage/stream_flash: Initialize settings at point of use

The commit moves Settigns initialization out of stream_flash
initialization function into: stream_flash_progress_clear
 stream_flash_progress_load and stream_flash_progress_save.

This slightly increases code size (~56 bytes on Arm) but allows
to initialize Stream Flash even if Settings subsystem fails
to initialize and continue providing its basic functionality.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>