First, I don't see any way that moving to OpenJDK is going to cost more than $10 billion. Second, if I understand correctly, OpenJDK has a license grant from Oracle. That's going to make it really hard for Oracle to say that Google doesn't have permission to use it.
Moving to OpenJDK might not, but does not retroactively ameliorate any issues they had.
I also suspect that OpenJDK's license might bite them in problematic ways, or they would have either done that to start or not had such difficulty negotiating an agreement under whatever terms Java possessed that Google might have wanted alternatives for.
> Moving to OpenJDK ... does not retroactively ameliorate any issues they had.
True. It does prevent damages going forward, however, and it removes Oracle's biggest club - a court order forbidding any further shipments of Android.
More than 9 billion? Give 36,000 people $250,000 a year salary and you still haven't hit 9.2 billion. Pretty sure they could get it done in a year with 36,000 people.
The only thing better than throwing more engineers at an already-late solution is hiring a mass swarm of engineers and turning them loose on a only-semi-bounded new problem.