In Android, like iOS, i use back button press for synchronize view - viewModel.
In Android, i use FormsApplicationActivity class and i override OnOptionsItemSelected method for calling physical method OnBackPressed.
Then inside ContentPage override, i detect back pressed and i can :
-Prevent close view by make a question
-Block closing view
When i change an android application : FormsApplicationActivity to FormsAppCompatActivity. this possibility disappear.
In fact, with FormsAppCompatActivity your renderer create Android ToolBar with FormsAppCompatActivity.ToolbarResource.
I never have possibility to intercept back click on this toolbar because your code call directly popAsync page...
Can you add possibility call OnOptionsItemSelected for intercept it and prevent closing page?