MaterialDateField DateChangedCommand Not Triggering

## 🐛 Bug Report

Binding a command to DateChangedCommand does nothing. This is a completely unresponsive command

### Expected behaviour
Command is triggered when the Date field has been changed

### Reproduction steps
add a MaterialDateField to your project and bind a command to the DateChangedCommand. Instantiate your command in your ViewModel and place a breakpoint on your receiving command's method. It will not trigger

### Configuration

**Version:** 1.7.7

**Platform:**
- [ ] :iphone: iOS
- [x] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [x] :monkey: Xamarin.Forms