Bluetooth: Make random number generation API public

Applications or higher level profiles that are not part of the core
Bluetooth stack may need an easy way to generate random numbers. Since
we already have an internal helper for this export it for others to
use as well.

Change-Id: I29af7cab30ad8f60d481bc847984e781eaecd6bf
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>