Switch to checking Grip Value instead of Grip Button

[BastiaanOlij]

Right now we're checking the grip button value to see if the user has is grabbing the controller.

This works well on Oculus controllers where the grip button is a button and will only read true if the user is holding the controller firmly.

Index controllers however have the grip set to true even when the player is lightly holding the controller making a lot of the grip based interactions fairly jittery to use.
Index controllers also give a grip value between 0.0 and 1.0, where only a firm grip reaches towards 1.0

Now OpenXR ensures that both options are available even for controllers that do not support one of the inputs.
So on Oculus we can use the grip value and it simply returns 0.0 when grip button is false, and 1.0 with grip button is true.

We should change our logic to use the grip value instead of grip button and toggle it based on a configurable threshold value.