Bluetooth: UUID: Expose bt_uuid_to_str to application

Expose the bt_uuid_to_str function as an API to the application.
This aligns this function with the bt_addr_to_str function call. This
allows the application to use this function without having to enable
the BT_DEBUG option.

Move the in-place bt_uuid_str to internal logging, this is mainly done
due to the limitation in the log_strdup that shouldn't be exposed to the
application.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>