Bluetooth: Mesh: Increase scan window from 10ms to 30ms

Experiments have shown that the probability of missing advertising
packets is significantly lower with 30ms scan window compared to 10ms
scan window. This is especially the case with advertisers using a 20ms
advertising interval, which in turn is perhaps the most common one
since it's the smallest allowed by the Bluetooth 5.0 specification.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>