Sound will sometimes not work on iOS13

[jbphet]

There is a change in iOS 13 where we are no longer able to use the user agent the same way we once did in order for the sim to be able to recognize that it is running on iOS 13. As a result, some sims can end up in a state where their sound isn't playing. The currently published versions of "Plinko Probability" and "Forces and Motion Basics" exhibit this problem. Here is the sequence to reproduce the problem on Plinko v1.1.17 (https://phet-dev.colorado.edu/html/plinko-probability/1.1.17/plinko-probability_en.html):

• launch the sim on an iOS 13 touch-based device (I got one from @KatieWoe, but it didn't have a name sticker on it the way most of the other devices do)
• without tapping, touch and drag somewhere on the screen that won't interact with anything
• select the "Intro" screen
• enable sound using the yellow speaker button
• drop a ball by pressing the play button

For me at least, this consistently failed to produce any sound, whereas the same sequence without the pre-interaction dragging works fine.

[jbphet]

The fix has been implemented and deployed in a maintenance release. Closing.