[android] fix memory leak in Window

[jonathanpeppers]

Fixes #15972
Context: https://github.com/markusroessler/MauiWindowScopeSample

In the above sample, android.permission.FOREGROUND_SERVICE is utilized to keep the app running even when terminated by the user.

Unfortunately, in this case Window & various other objects live forever. When running the sample, the InstanceCount grows each time you close & launch the app again:

07-07 16:08:32.281 22562 22562 I DOTNET  : ScopedService InstanceCount: 3
07-07 16:08:32.291 22562 22562 I DOTNET  : MainWindow InstanceCount: 3

The first issue I noticed was IWindow.Destroying() was never called because of this code in OnDestroy():

if (!activity.IsFinishing)
{
    var window = activity.GetWindow();
    window?.Destroying();
}

IsFinishing was always true from my debugging in this spot.

OnStop() had the code:

// As of Ice Cream Sandwich, Stopped is guaranteed to be called
// even when the activity is finishing or being destroyed
// We check for finishing and call destroying here if so
if (activity.IsFinishing)
    window?.Destroying();

In this case IsFinishing was false.

I think the solution here is that OnDestroy() should attempt to destroy the Window in all cases. There isn't a case you would not want to destroy it. The only problem being we can now destroy it twice -- and there is no IWindow.IsDestroyed value to check. We can simply handle InvalidOperationException, as this should be rare.

With this change in place, I still saw a leak. Reviewing a .gcdump of the app, I saw the object tree:

Microsoft.Maui.Controls.Platform.AlertManager.AlertRequestHelper
    List<Microsoft.Maui.Controls.Platform.AlertManager.AlertRequestHelper>
        Microsoft.Maui.Controls.Platform.AlertManager
            MauiWindowScopeSample.MainWindow
    Microsoft.Maui.Controls.MessagingCenter.MaybeWeakReference
        Microsoft.Maui.Controls.MessagingCenter.Subscription
            List<Microsoft.Maui.Controls.MessagingCenter.Subscription>
                Dictionary<Microsoft.Maui.Controls.MessagingCenter.Sender, List<Microsoft.Maui.Controls.MessagingCenter.Subscription>>
                    Microsoft.Maui.Controls.MessagingCenter
                        Other Roots

Where MessagingCenter appears to keep AlertRequestHelper alive, which keeps the Window alive.

After these changes, we instead get this every time:

07-07 16:08:38.967 22562 22562 I DOTNET  : ScopedService InstanceCount: 2
07-07 16:08:38.978 22562 22562 I DOTNET  : MainWindow InstanceCount: 2
07-07 16:08:39.015 22562 22562 I DOTNET  : Lifecycle event: OnStart
07-07 16:08:39.072 22562 22562 I DOTNET  : Lifecycle event: OnPostCreate
07-07 16:08:39.073 22562 22562 I DOTNET  : Lifecycle event: OnResume
07-07 16:08:39.074 22562 22562 I DOTNET  : Lifecycle event: OnPostResume
07-07 16:08:39.411 22562 22585 I DOTNET  : MainWindow InstanceCount: 1
07-07 16:08:39.414 22562 22585 I DOTNET  : ScopedService InstanceCount: 1

The InstanceCount returns to 1 as the finalizers run for these types.

I have yet to figure out how to write a test for this scenario -- I've only been able to validate this fix manually testing the app.

[jonathanpeppers]

This call unsubscribes from MessagingCenter things on Android.