Android back button behavior in a modal on Xamarin Forms
## 🐛 Bug Report

When navigating to a second page in a modal on Xamarin Forms, the device's (hardware) backbutton behaves differently than the backbutton on the top left of the screen and when compared to iOS and UWP.

### Expected behavior
The device's backbutton on Android should behave the same as the back button on the top left on the screen in a modal, navigating up the navigation stack in stead of closing the modal.

**Comparison:**
| Platform | Going back with... | Result |
|-----------|--------------------|--------|
| UWP | Back button on the top left in the window's title bar | ✅ Goes back to the previous screen inside modal |
| iOS | Back button on the top left in title bar | ✅ Goes back to the previous screen inside modal |
| Android | Back button provided by the app (top left) | ✅ Goes back to the previous screen inside modal |
| Android | Back button provided by the system (hardware/software back key or system-wide gestures) | ❌ Closes modal |

### Reproduction steps
1. Open a modal
2. Navigate to a second screen inside this modal
3. Press the device's (hardware) back key.
Step 3 will close the modal in stead of going back to the first screen in the modal.

I have made a [sample project](https://github.com/patrick11994/MvxXFModalNavigation) to demonstrate this.

### Configuration

**Version:** 7.x

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