There is an AI driven optimizer in play[1], but what is optimised is what has changed. What is far more important now is optimising programmer time, time to market, testability etc. Using less CPU and RAM is largely irrelevant until the usage is actually noticeable by the user - taking 50 milliseconds instead of 70 milliseconds to do a spell check is not something any user can even detect. It would be a huge waste of programmer and purchasing of tools to focus on that instead of all the other things that need to be developed and made available to users.
[1] The optimizer is a human brain, or a collection of them if this isn't a lone wolf project
Time to market and developer productivity are still in the forefront. This is why you repeatedly see apps being released, user outcry and then later revisions trying to address those concerns (battery, cpu, memory, responsiveness etc). I doubt anyone actually maximizes idle time, but rather decreases user complaints and improve acceptance. There is no need to maximize it if users don't notice.
[1] The optimizer is a human brain, or a collection of them if this isn't a lone wolf project