OK Google != Hey Siri.
Update: Google took a huge step forward. (Do they read my blog!?) You can now do OK Google from any screen. Yes, even a locked PIN screen. It’s clunky but it works. Now, they need to get Google Now closer to Siri in terms of common things like directions and make it less specific with keywords. Siri is still much better for common language commands.
Most Android phones cannot do much from a PIN locked screen. A good exception is the Moto X which really can do this from a PIN locked screen and indeed can implement this feature while sleeping.
In iOS8 with the available hardware as of this date, can only do this while plugged in.
The OnePlus One is rumored to do this, but hardly anyone has one of them.