This is an old question, but one thing to keep in mind that nobody else seems to have mentioned is that if an action is typical and potentially frequent, a toast should be kept away from the action. Otherwise it could be obtrusive and could prevent users from accomplishing their task.
A toast should provide helpful feedback but should not be required. You say that they're used on Android for "confirmation", but more accurately they should be used for feedback that something happened. If a user sees the toast, great. If a user doesn't, then that should be okay too. IMO, most toasts should be kept out of the way.
Did Google consciously position the toast here for optimal usability?
Depends on what you're optimizing for. If you're optimizing for allowing users to accomplish their task (which is often what is meant by "usability"), then I think near the bottom is a reasonable location. If you're optimizing for visibility, then it's not. However, if you need to display an important, severe message, a toast would be inappropriate, especially since toasts are ephemeral.