Need to save selected items on clicking native android back button?

I use Shell Navigation in my project.

I overrride BackButton in xaml(PCL) and add BackCommandBehavior.

My problem :
If i select some fields and go back using BackButton (loaded on top left corner) - selected item saves.
and
if i go back using NativeAndroidBackButton in device - select item does not save and display in upcoming page.

My Try:

PCL : Added dependency service in OnNavigatedTo of upcoming page.
Android :
override OnBackButtonPressed method in renderer and enable bool property.
In implemented dependency service, i returned that bool property.
PCL :
Once call returned form Android renderer, based on bool property i invoke BackCommand return in PCL.

But my bool property never returns true...always false.

Why?
Any possiblities?