Fix navigation stuck on iOS when pushing on hidden tab section in TabBar

[albyrock87]

Description of Change

On iOS, the view controller DidShowViewController will not be triggered until the Tab inside TabBar becomes visible.
The code is waiting on that before switching to the new Tab, this causes a race condition.
So in this specific case, we have to complete the task immediately instead of waiting for DidShowViewController.

Issues Fixed

Contributes to #16568 by fixing the scenario when the number of tabs is <= 5 (a.k.a. no "More" tab).
See my comment below regarding "More" tab.

[dotnet-policy-service]

Hey there @albyrock87! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jsuarezruiz]

/azp run

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[albyrock87]

Build is failing for an unrelated test:

Error Message:
   Shape should not be alive!
  Stack Trace:
     at Microsoft.Maui.UnitTests.Views.ShapeTests.ShapeFillDoesNotLeak() in /_/src/Core/tests/UnitTests/Views/ShapeTests.cs:line 46

[albyrock87]

Closed in favor of #22655

[PureWeen]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[albyrock87]

So here's my discovery regarding #16568 and #18193.

We're using UITabBarController as ShellItemRenderer.
Each view controller in ViewControllers is a ShellSectionRenderer which inherits from UINavigationController (in order to handle the navigation stack).

Everything's fine until we hit the limit (N>5). In this case MoreViewController enters the game.
When the user presses on the More icon, ShellItemRenderer.SelectedViewController becomes UIMoreNavigationController where the root view controller represents the list of shell sections (ViewControllers) hidden inside.

The problem starts when the user taps on one of the sub-pages list item: UIMoreNavigationController (which is a UINavigationController itself) should push ShellSectionRenderer into its stack, but what happens is that it transfers ShellSectionRenderer stack inside UIMoreNavigationController to avoid nesting UINavigationControllers (see: https://stackoverflow.com/a/11967127).

At this point ShellSectionRenderer.ViewControllers becomes empty, which obviously creates a very odd situation.
The stack is then transferred back when the user leaves the MoreViewController.

It seems there's no way to fix this besides creating our own "more" mechanism.