My dog also has this and her doctors say that this is completely normal and some shih tzus develop those spots on their bodies and they usually develop in areas where the shih tzu's different colored hair patches are. My dog has developed many even though at first she never had any. Don't worry.
In my unprofessional opinion, I don't think the spots are anything to worry about. Like people, dogs can develop differently pigmented areas on their skin, and it's just the way they were meant to be and entirely normal. Since you're concerned, bring it up with your vet on her next checkup.
As for the paw licking, she may be doing that out of simple boredom. When dogs don't get enough exercise, one thing they do to relieve the boredom of living the unnatural life of a couch potato is begin obsessively licking one part of the body. Over time, a bald spot develops that is commonly referred to as kennel callus. Frequent licking of the paws keeps them constantly wet, and that can develop into a yeast/ fungal infection that is very itchy and uncomfortable - just like athlete's foot in a human. Yeast infections are easily treated with preparations you can get from your vet, but you'll need to find a way to stop the licking so it doesn't return. If you aren't already walking at least once a day, it's time to start. :-)