subsys/settings: Cleanup the initialisation of the subsys

This commit removes redundant checks if the module
was already initialized.
The variable to mark the fact of initialization is
moved as a global module variable.
This allows creating more sophisticated unit tests
of the settings subsystem by giving a possibility to modify
the internal mark of the fact the system was initialized.

Signed-off-by: Radoslaw Koppel <radoslaw.koppel@nordicsemi.no>