Familiarity and learnability are not the only rubrics for UI design.
While it's good to make products understandable by two-year-olds, there are other important considerations in UI. Can it help you accomplish the task efficiently? Reliably? Happily? Vim, for example would never stand up to the two-year-old test, but is nonetheless an extraordinarily capable and efficient text editor--which qualifies it, in my opinion, for "good UI".
I don't mean to imply Android is as readily cognizable to toddlers as the iPhone. What I mean to say is that streaming quickly from activity to activity through various stateless apps, each of which can plug into each other for various intents, is a more efficient way to accomplish tasks than quitting and re-opening apps from the home screen.