Disable back button

[LukasCarton]

When a user is tracking his route (TrackingPage), it shouldn't be possible to go back to the MainPage.
When a user views the summary of the bike ride (TrackingSummaryPage), it shouldn't be possible to go to the TrackingPage.
At the moment, those 2 things are possible.

Links:
https://forums.xamarin.com/discussion/113715/is-there-a-way-to-override-a-back-button-press-on-android-and-ios-in-xamarin-forms#latest