Humans don't carry nukes because fissile material is _heavy_. Add onto that the shielding to make sure the person can even carry it without becoming gravely ill very shortly into their delivery and the fact a suitcase nuke or dirty bomb has never been personally delivered is not very surprising.
The ridiculous part is that making an android carry this payload doesn't change the nature of the payload. Heavy, fissile material will still give off signatures that will trip all manner of alarms.
Fatman, the plutonium implosion bomb that was detonated over Nagasaki, used only 6.2kg of Pu. Also, the Pu-239 used in weapons doesn't require a lot of shielding. There are plenty accounts of people handling nuclear weapon cores that have had only a light plating of other metals or with only thick gloves.
What probably is quite heavy is all of the associated components that you need to make a bomb - the chemical explosives, tamper etc.
The W54, one smallest nuclear warhead anyone is admitting to making, was about 23kg - although this did have a very small yield.
There are some much lighter elements you can use. curium-247 would be ideal.
A long half life (so you can store it), the radiation is all alpha particles, which are undetectable remotely, and is safe to handle (no shielding necessary, just cover it). (However the decay products are much more dangerous, so it's not a total panacea, fresh curium would be desired.)
It has a critical mass of about 7Kg, so it's easy to carry.
Of course the hard part is making it.....
You also have to refine it carefully, since all the contaminants are very radioactive. (However mixing other isotopes of curium with it is not a serious problem, so refining it is not incredibly hard.)
The ridiculous part is that making an android carry this payload doesn't change the nature of the payload. Heavy, fissile material will still give off signatures that will trip all manner of alarms.