[Enhancement] Expose AccessibilityValue on iOS (and equivalents on other platforms)

_From @johnshardman on Wed, 18 Nov 2020 11:35:32 GMT_

The AutomationProperties.NameProperty and AutomationProperties.HelpTextProperty are useful for naming controls and adding help text. Using them to expose a control's value results in accessibility guidelines being breached on target platforms, and result in a poor user experience for those using accessibility options/devices (such as Braille readers). For example, if using VoiceOver on iOS, the default behavior for a Slider is that the position of the Slider is read out as a percentage. In most situations (in my experience at least), that's not what the user wants. The user wants the value read out, not the percentage. So, if the Slider runs from 1 to 365 for days in the year, the user might want to hear 180 rather than 50%. Using AutomationProperties.NameProperty to read out the 180 has multiple drawbacks (for a start, it doesn't stop the 50% being read out first).

To solve this, it's necessary to somehow expose the value (in the case of iOS, AccessibilityValue). I suggest adding AutomationProperties.ValueProperty as a bindable property - that would be consistent with the other properties, although there are other ways of exposing it. This should be done for all controls, not just Sliders.

_Copied from original issue xamarin/Xamarin.Forms#12889_

Start by tracing AutomationProperties.NameProperty and HelpTextProperty into the platform accessibility mappings, beginning with iOS AccessibilityValue. Define how a value property should work across controls and platforms, then verify that controls expose the value without replacing their standard accessibility behavior.

Written by the indexing model from the issue text.