Update where we setup BackHandler for Android

[kierse]

Problem

#433 (part of efforts to improve KMP support) renamed BaseNavigableCircuitContent to NavigableCircuitContent, overloading the existing Android-specific method with the same name. The only difference between the two method signatures was an additional param in the android version (related to setting up the BackHandler).

As a result of this change existing uses of NavigableCircuitContent on Android started calling the wrong method, bypassing the logic that configured the system back handler.

Fix

Rather than change the Android overload in some way, or require that Android callers provide the extra parameter (a friction when 99% of uses will want the default), I opted to move the back handler setup logic into the Android-specific rememberCircuitNavigator, and remove the Android overload entirely.

[ZacSweers]

Nice solution 👍

[ZacSweers]

Actually, I think rememberSaveableBackstack may also get moved to common eventually so this may still be an issue in the future. I do think this is a better place for this API though 👍. Maybe when that time comes we just make this an expect/actual setup