AccessibleSlider doesn't work with mobile VoiceOver

[jessegreenberg]

Found while working on phetsims/scenery#852, AccessibleSlider doesn't work with gesture input on mobile VoiceOver.

It isn't receiving any input events when flicking up and down on the screen when VO is enabled. I discovered that when this line is removed, the problem goes away:

// value 'any' allows input to have values that are not evenly divisible by the step size, which is
// required for PhET sliders, see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#attr-step
this.setAccessibleAttribute( 'step', 'any' );

[jessegreenberg]

From https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#attr-step

If this attribute is not set to any, the control accepts only values at multiples of the step value greater than the minimum.

Ill see if step="any" works in mobile safari/VO outside of a PhET sim. If it does, we can troubleshoot this use case. If not, maybe there is another way to support range inputs with values that are not evenly divisible by the step size.

[jessegreenberg]

Well... if step=any, how would the browser know how much to change the value by from a gesture?
EDIT: I guess the same is true for key input, so the attribute assumes the developer will handle the value. But there is no way to detect gesture input while AT is active.

[jessegreenberg]

Working with the above commits. I removed the any value for the step attribute, instead just using a value that evenly divides the range. Also, there was an issue in scenery where the value for the primary sibling wasn't getting updated correctly, that was fixed as well. Closing.