Android is ultimately intended for OEMs. If Google was not dealing with corporations that have billions of dollars at stake, they might be more likely to be more open. But honestly, the difference is the product. A browser is free to the final customer, has no real hoops to jump through before release, and since all browser makers have been heading to the same basic goal for over a decade and (mostly) adhere to a standard, there is not really anything to gain from secrecy. A phone/tablet costs money to the final user, has to go through OEMs, carriers, and the Feds before it can be in anyone's hands. Not to mention that we are in the infancy of mobile computing and secrets can give a real advantage. I wouldn't be surprised if they are much more open in five years.

Basically, I'd like them to be more public, but there are real reasons not to be currently and a code dump is better than nothing.

The Chrome team doesn't fully work in public either. For example, Crankshaft was developed in secret and then dumped into the bleeding-edge branch on the day it was announced.

For each Android release one phone vendor seems to get an exclusive; presumably they are paying for that in some way (like TV ads that promote the platform).

I don't have an answer, but it really undermines their "openness" argument.

Related question, if someone knows: Does the Android team take patches from those outside Google? (I know that Chrome does.)