[Xamarin.Android.Build.Tasks] lazily populate Resource lookup

[jonathanpeppers]

Fixes: #7684

Comparing .NET 7 to main, I noticed:

.NET 7
Task LinkAssembliesNoShrink 4ms
main/.NET 8
Task LinkAssembliesNoShrink 101ms

Under dotnet trace a lot of the time was spent in:

94.74ms MonoDroid.Tuner.FixLegacyResourceDesignerStep.LoadDesigner()

Reviewing the code, I think we can "lazily" call the LoadDesigner()
method. Now it delays until the ProcessAssemblyDesigner() method. I
had to reorder logic slightly to do this.

I also updated one log message to only log duplicates, as it was
logging hundreds of lines:

if (output.ContainsKey (key)) {
    LogMessage ($"          Found duplicate {key}");
} else {
    output.Add (key, property.GetMethod);
}

Which also showed up in dotnet trace:

25.58ms Microsoft.Android.Build.Tasks.MSBuildExtensions.LogDebugMessage()

With these changes, I instead get:

Task LinkAssembliesNoShrink 5ms

Which is probably ~the same performance as before or plenty good enough!

Lastly, I also removed some other code & members in
FixLegacyResourceDesignerStep that Visual Studio showed me was
unused.

[jonathanpeppers]

I think it's going to work this time, my first attempt triggered the failure:

ApplicationRunsWithoutDebugger(False,False,False)
Activity should have started.
Expected: True
But was:  False

So we should have decent test coverage on this.

[dellis1972]

This might have been an ovesight on my part, but I'm not sure we have a test which specifically tests this senario on device.
i.e an app which does use the new designer system with a library that does not. We might need to add one or modify an existing one to do that.

Problem is the test would need to be DotNet only since the new system is not supported.

[jonathanpeppers]

When I was debugging ApplicationRunsWithoutDebugger, I found it was actually testing this scenario (why I got a failure with my first attempt 30501c1 before when I broke something):

https://github.com/xamarin/xamarin-android/blob/8b0531465d90f6357bd9271faf7e5d6f7f3e4a46/tests/MSBuildDeviceIntegration/Tests/DebuggingTest.cs#L48-L57

Because this app is Xamarin.Forms (which uses Resource.designer.cs in a class library), I was getting an InvalidCastException on startup when this was wrong. It had an incorrect integer for a resource id, which caused the exception at startup.