Android embedding: android.app.Fragment is deprecated. Switch to support Fragment.

[atsushieno]

Starting from Android P, android.app.Fragment is officially deprecated: https://developer.android.com/reference/android/app/Fragment.html

We are using Android.App.Fragment in our Forms Embedding. We should switch to support Fragment too. Not even for the official deprecation, Support Fragment is getting increasingly important as it supports a lot of features that non-support Fragment does not (such as Architecture Components).

https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Platform.Android/AppCompat/PageExtensions.cs#L2