Is there a hands free way to manipulate pandora while driving?
This would really have to be enabled by the developers of the app, unfortunately. An always listening app would be very risky to distribute, as it would generate many complaints about battery life...

I have an Android Wear watch, which makes interaction minimal and on your wrist as you drive. This is my best suggestion, though this also would depend on the developers, if they have implemented Wear functionality. 

I know this is a more costly solution than an app, but there's more to it than just this functionality. Wear apps can also implement voice functionality, so those developers better get working...