Fix Shell Android navigation icon flicker during push/pop animation

[Copilot]

On Android Shell with a Flyout, pushing a page causes the hamburger icon to briefly appear during the animation before switching to the back arrow once the animation completes. The same flicker occurs on pop.

Root Cause

PR dotnet#28187 (Issue21646 fix) introduced a regression in ShellToolbarTracker.UpdateLeftBarButtonItem. The icon-setup conditions were changed to use backButtonVisible = _toolbar.BackButtonVisible, but this value is copied from the global toolbar at OnCreateView time — which reflects the previous (root) page's stale state (BackButtonVisible = false). The global toolbar isn't updated until shell.Navigated fires after the animation completes.

Fix

Replace backButtonVisible with backButtonVisibleFromBehavior in the four icon-setup conditions in ShellToolbarTracker.cs:

// backButtonVisibleFromBehavior reads directly from BackButtonBehavior.IsVisibleProperty
// on the destination page — correct at fragment creation time, before the toolbar syncs

if (icon == null && (_flyoutBehavior == FlyoutBehavior.Flyout || (CanNavigateBack && backButtonVisibleFromBehavior)))
icon?.Progress = (CanNavigateBack && backButtonVisibleFromBehavior) ? 1 : 0;
if (command != null || (CanNavigateBack && backButtonVisibleFromBehavior))
    if (backButtonVisibleFromBehavior || !defaultDrawerArrowDrawable)
        toolbar.NavigationIcon = icon;

Both variables agree after navigation completes, so post-animation behavior is unchanged. The Issue21646 case (BackButtonBehavior.IsVisible = false → hamburger stays visible) continues to work correctly.