shell: Fix shell init procedure when configured as inactive on startup

The previous behavior of the CONFIG_SHELL_AUTOSTART option, where setting
it to 'n' disabled shell interaction at startup but kept the logger
active as a shell backend, was inconsistent.
Now, when CONFIG_SHELL_AUTOSTART is set to 'n', both the shell and the
logger are inactive at startup. Calling the shell_start function will
activate them and print any pending logs. This change ensures a more
consistent and logical behavior regarding shell and logger activity at
startup.

Additionally, now when the shell_stop function is called, both the shell
and the logger are halted. This enhancement ensures that stopping
the shell also effectively stops the associated logger.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>