I thought Oracle was pretty clear that they bought Sun for Java.
But I didn't say patent trolling was the reason Oracle bought Sun. I said that could be the long-term value of the purchase.
And I certainly don't see how Android is "essentially a closed platform". It's closed to the extent that it needs to be in order for it to be acceptable to the carriers, but is much more open than the iPhone or any other widespread (at least in the US) smartphone OS. They keep some apps and drivers to themselves, and they aren't incredibly accepting of outside contributions, but the code is out there for people to modify and add to.
I doubt that Oracle cares whether the FOSS community abandons Java, since the FOSS community (unlike the enterprise world) never really embraced Java very much. Google itself is pretty tied to Java though.