Mobile VoiceOver problem when "step" attribute is too small

[jessegreenberg]

Discovered while working on #499, and required for phetsims/gravity-force-lab-basics#119. <input type="range"> doesn't receive any input events from VoiceOver when the step attribute is too small relative to the range of the slider. In gravity force lab basics, the step attribute is 1/1000000000 of the range. If the step attribute is made larger to be 1/100000 of the range, input events are received. Currently, the step attribute is determined by

          // The step attribute must be non zero for the accessible input to receive all accessibility events, and
          // only certain values are allowed depending on the step size, or else the AT will announce values as
          // "Invalid". If the step size is equal to the precision of the slider readout, all values are allowed.
          this.setAccessibleAttribute( 'step', Math.pow( 10, -this.a11yDecimalPlaces ) );

This line was determined to be the best in #413 for non-mobile AT. We should return there for history.

[jessegreenberg]

This isn't working because the step is using different units than the physical Property in GFLB. step is in billions of kg, while the range is in kg.

[jessegreenberg]

I tried step="any" again (which is a valid DOM attribute) from #413, but it does't work with iOS VoiceOver either.

[jessegreenberg]

I workaround has been added with the above commit, along with a lot of documentation that describes PhET's slider conquest until this point. Closing this issue, since discussion can continue in the related #495