Bluetooth: controller: Scanning with unreserved window

Added a Kconfig option to enable scanner with unreserved
scan window when in continuous scan mode. This will permit
scanner to be always pre-empted by other roles, making it
the lowest in priority when it comes to using the radio.

Fixes #27414.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>