[iOS] Fix inability to check an initially disabled RadioButton after enabling it

[lee-m]

Description of Change

The RadioButton control used on iOS was not listening for changes to the IsEnabled state to add the tap gesture recogniser needed to toggle the control's checked state.

UpdateIsEnabled was previously only called once when the control was first created, so if a RadioButton was created in an initially disabled state, there was nothing in place to add the tap gesture recogniser at a later point. This PR listens for changes to IsEnabled via OnPropertyChanged to call UpdateIsEnabled which will add/remove the tap gesture recogniser based on the enabled state.

This doesn't affect Android as that uses the built in radio button views rather than the Xamarin Forms implementation of one.

Issues Resolved

Fixes #14544

API Changes

None

Platforms Affected

• iOS

Behavioral/Visual Changes

Before, adding a RadioButton control to a page with IsEnabled set to false meant that RadioButton did not respond to taps after changing IsEnabled to true. After this change, setting IsEnabled to true allows the RadioButton to respond to taps as expected to toggle the checked state.

Before/After Screenshots

Not applicable

Testing Procedure

• Add a RadioButton to a page with IsEnabled set to false.
• After the page has loaded, change IsEnabled to true (via a button click etc.)
• Verify that the RadioButton's checked state can now be updated via tap gestures.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[net-foundation-cla]

All CLA requirements met.

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jfversluis]

Changed the test case a little, but other than that looks good!

[jfversluis]

Thanks for your very first contribution @lee-m ! Hope you're filled with pride because you should be! Go forth and call yourself a Xamarin.Forms contributor!