Can you train a small flying animal to activate and drop grenades on stormtroopers? like the Spintari Glowbat?

You can direct your Bonded pet using a Manoeuvre to perform a Manoeuvre and an Action. Dropping a grenade would fit in there I think.

There are no rules for training a Beast skill ranks, but over an extended campaign I could see a beast learning some tricks with extensive training.

So for me yes, I would allow it, but with a few common sense restrictions. The small animal could only carry a single grenade or it gets overloaded and can't fly. It also uses its own Ranged Light skill so it probably has no ranks.