True. But Play Services is effectively a part of the OS (from Google's perspective).
As you say, in both cases (iOS and Play Services) it's a commercial closed-source bundle. shrug
I don't personally spend time worrying about that, given that Google and Apple's code is probably better reviewed than some random open source app, but some people like to nerd out about such things.
As you say, the FBI was eventually able to get access to the San Bernardino shooter's phone. But this isn't exclusive to the highest levels of government; it just depends on your budget: http://www.reuters.com/article/us-apple-encryption-fbi-idUSK.... It's not surprising the CIA would have a stockpile of unpatched 0days, found or bought.
I don't believe I'm worth $1m to anyone, so I feel pretty safe using both iOS and a recent, patched Android.