There was a commit in the AOSP with a note asking for change in mnc-dev branch.

MNC could be code for the next Android version, and hence Macadamia Nut Cookie would fit.
Though Google's done this before and actually changed the project's nickname, fooling everyone.
I wouldn't be surprised if it happened again. Though I doubt they'll do another promotional one like with Nestle's Kit Kat brand, so M&Ms is out of the picture.

To summarize possible codenames:
- Macadamia Nut Cookie
- Marshmallow
- Muffin
- Mooncake
- Mint Cookie
- Mocha Mousse/Trifle
- Mango Sorbet/Pudding
- Maple Syrup (Maple also a dig at Apple)

I personally believe "Marshmallow" is the most likely one considering the previous Jelly Bean, Kit Kat, and Lollipop monikers. They're not only all desserts, but they're all junk food.