fix: error "Navigator ... is replacing an already attached ..."

[hoc081098]

When changing startRoute, that causes navController.navigatorProvider.addNavigator(CustomActivityNavigator(context)) is called again.
check(previousNavigator?.isAttached != true) don't passes.

package androidx.navigation

public open class NavigatorProvider {
 @CallSuper
    public open fun addNavigator(
        name: String,
        navigator: Navigator<out NavDestination>
    ): Navigator<out NavDestination>? {
        ...
        val previousNavigator = _navigators[name]
        if (previousNavigator == navigator) {
            return navigator
        }
        check(previousNavigator?.isAttached != true) {
            "Navigator $navigator is replacing an already attached $previousNavigator"
        }
       ...
    }
}

P/S:
Although changing startRoute does not follow Fixed start destination, but in some cases, we must to do this.

[hoangchungk53qx1]

@hoc081098 💯

[gabrielittner]

Thanks for this PR and the other one, I'll take a loot at them this week

[gabrielittner]

LGTM. We should probably turn the Set parameters into ImmutableSet at some point to make he effect and remember more efficient