aria-valuetext doesn't update interacting with mouse

[zepumph]

Discovered as part of phetsims/scenery#951. I would expect the Property passed in to AccessibleSlider to update appropriately no matter what the input modality is, but it seems like the Property only changes when using a keyboard, and thus, only the aria-valuetext in the PDOM updates when using the keyboard.

I don't know much about this sim, but I figured this deserved a low-ish priority issue just to track the bug.

[zepumph]

Tagging @jessegreenberg so he is aware.

[jessegreenberg]

Hmm, this seems like it would be a general issue for all things that use AccessibleSlider, but Ill take a look as part of phetsims/scenery#951 review.

[jessegreenberg]

It looks like not just aria-valuetext but slider value as well. I confirmed that this is not an issue for other things using AccessibleSlider.

[jessegreenberg]

It is because the AppendageNode gives the AccessibleSliider a Dynamic bidirectional Property to map from angle to input value along the slider ranges.

[jessegreenberg]

Fixed with the above commit.