FIX: Fixed incorrect default state for axes on some controllers.

[jechter]

When a control does not have an explicit defaultState argument to the InputControl attribute, the default state is considered to be 0. But for axis controls which are normalized to return values from -1..1, and which are represented by an unsigned integer on the device, 0 is not the correct default value for the input state. This meant that the axis would be considered to be at it's default value when at -1. This is common for gamepad access, and would result in canceled callbacks being sent when moving the axis towards -1.

Now, in combination with my previous fix here: https://github.com/Unity-Technologies/InputSystem/pull/828/files#diff-a82dc3439d6b317a87e69ff2cdc0bc0a , which would always return 0 values for canceled interactions, this would mean that moving a gamepad stick all the way to the left would generate a value of 0, which completely broke the Input System.

This PR automatically generates default values for axes when no default is explicitly set and when normalizeZero is used, which fixes the problem. This is the best fix I could think of - I also considered adding default values everywhere (but that would have been much work, and easy to get wrong or miss a place), or to ignore any controls without explicit default values for CheckStateIsAtDefault, but that would skip optimizations and possibly change behavior).