[android] improve performance of Shell flyout opening

[jonathanpeppers]

Context: https://github.com/dotnet/maui/issues/10713 Context: https://github.com/supershopping/ShellFlyoutLagging

In reviewing the above customer sample, there is a noticeable delay on some devices when you open a Shell flyout for the first time.

This was pretty close to a default Shell setup:

<Shell Shell.FlyoutBehavior="Flyout">
    <FlyoutItem Title="Main Page">
        <ShellContent Title="Main Page"  Route="MainPage" ContentTemplate="{DataTemplate local:MainPage}" />
    </FlyoutItem>
    <FlyoutItem Title="Test Page">
        <ShellContent Title="Test Page"  Route="TestPage" ContentTemplate="{DataTemplate local:TestPage}" />
    </FlyoutItem>
</Shell>

Reviewing the code, there was an oddity where we create & replace a RecyclerView's LayoutManager:

SetLayoutManager(_layoutManager = new ScrollLayoutManager(context, (int)Orientation.Vertical, false));
SetLayoutManager(new LinearLayoutManager(context, (int)Orientation.Vertical, false));

We think this is likely something that went wrong during a git merge, etc.

In addition to removing code, I create a new ShellRecyclerView in Java that reduces the amount of interop calls from C# to Java. We can do all the work now in the ShellRecyclerView's constructor:

var adapter = new ShellFlyoutRecyclerAdapter(ShellContext, OnElementSelected);
var recyclerView = new ShellRecyclerView(context, adapter);

dotnet-trace output shows these changes have a reasonable impact on a Pixel 5 device:

Before:
35.34ms microsoft.maui.controls!Microsoft.Maui.Controls.Platform.Compatibility.ShellFlyoutTemplatedContentRenderer.CreateFlyoutContent()
After:
17.64ms microsoft.maui.controls!Microsoft.Maui.Controls.Platform.Compatibility.ShellFlyoutTemplatedContentRenderer.CreateFlyoutContent()

Saving about ~17.7ms of time opening the Shell flyout on Android.

[jonathanpeppers]

I'm able to add so many <FlyoutItem> that scrolling would be required, it appears to still scroll fine:

I don't think these changes break anything, but I could test something else, let me know.

[jonathanpeppers]

Let me rebase and rebuild maui.aar.

[rmarinho]

/rebase

[jonathanpeppers]

At this point, maybe I manually rebase this, rebuild maui.aar, and target the net9.0 branch instead?

This doesn't really feel like a "servicing" PR, because it is performance improvements.