[trimming] fix custom applications for TrimMode=full

[jonathanpeppers]

Fixes: #8914
Fixes: #7301

Using a custom application with TrimMode=full:

[Application]
public class CustomApp : Application
{
    public CustomApp(IntPtr h, JniHandleOwnership o) : base(h, o) { }
}

Fails with:

ILLink : error IL1012: IL Trimmer has encountered an unexpected error. Please report the issue at https://aka.ms/report-illink
Fatal error in IL Linker
Unhandled exception. System.InvalidOperationException: Sequence contains no matching element
    at System.Linq.ThrowHelper.ThrowNoMatchException()
    at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
    at MonoDroid.Tuner.PreserveApplications.PreserveTypeProperty(CustomAttribute attribute, String property) in /Users/runner/work/1/s/xamarin-android/src/Microsoft.Android.Sdk.ILLink/PreserveApplications.cs:line 69
    at MonoDroid.Tuner.PreserveApplications.PreserveApplicationAttribute(CustomAttribute attribute) in /Users/runner/work/1/s/xamarin-android/src/Microsoft.Android.Sdk.ILLink/PreserveApplications.cs:line 60
    at MonoDroid.Tuner.PreserveApplications.ProcessAttributeProvider(ICustomAttributeProvider provider) in /Users/runner/work/1/s/xamarin-android/src/Microsoft.Android.Sdk.ILLink/PreserveApplications.cs:line 55
    at MonoDroid.Tuner.PreserveApplications.ProcessType(TypeDefinition type) in /Users/runner/work/1/s/xamarin-android/src/Microsoft.Android.Sdk.ILLink/PreserveApplications.cs:line 43
    at MonoDroid.Tuner.PreserveApplications.<Initialize>b__0_1(TypeDefinition type) in /Users/runner/work/1/s/xamarin-android/src/Microsoft.Android.Sdk.ILLink/PreserveApplications.cs:line 21
    at Mono.Linker.Steps.MarkStep.MarkType(TypeReference reference, DependencyInfo reason, Nullable`1 origin)
    at Mono.Linker.Steps.MarkStep.MarkTypeVisibleToReflection(TypeReference type, TypeDefinition definition, DependencyInfo& reason, MessageOrigin& origin)
    at Mono.Linker.Steps.MarkStep.MarkEntireType(TypeDefinition type, DependencyInfo& reason)
    at Mono.Linker.Steps.MarkStep.MarkEntireAssembly(AssemblyDefinition assembly)
    at Mono.Linker.Steps.MarkStep.MarkAssembly(AssemblyDefinition assembly, DependencyInfo reason)
    at Mono.Linker.Steps.MarkStep.MarkModule(ModuleDefinition module, DependencyInfo reason)
    at Mono.Linker.Steps.MarkStep.ProcessMarkedPending()
    at Mono.Linker.Steps.MarkStep.Initialize()
    at Mono.Linker.Steps.MarkStep.Process(LinkContext context)
    at Mono.Linker.Pipeline.ProcessStep(LinkContext context, IStep step)
    at Mono.Linker.Pipeline.Process(LinkContext context)
    at Mono.Linker.Driver.Run(ILogger customLogger)
    at Mono.Linker.Driver.Main(String[] args)
    at Mono.Linker.Driver.Main(String[] args)
c:\Nuget\microsoft.net.illink.tasks\9.0.0-preview.3.24172.9\build\Microsoft.NET.ILLink.targets(91,5): error NETSDK1144: Optimizing assemblies for size failed.

I could update an existing CustomApplicationClassAndMultiDex test to reproduce the problem.

What is happening is:

• TrimMode=full makes the trimmer way more aggressive.

• Android.App.ApplicationAttribute has various unused members trimmed away.

• BackupAgent and ManageSpaceActivity properties are trimmed away.

• Custom trimmer step throws an exception when trying to access these trimmed properties.

The trimmer step already handles cases where it can't load the types and happily continues on. We should just do the same if the properties are not found: as it would mean a customer didn't set them.

As a workaround, @tranb3r tried:

[global::Android.App.ApplicationAttribute(
    ...
    BackupAgent = null,
    ManageSpaceActivity = null
)]

But this results in a NullReferenceException in ManifestDocumentElement:

C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\34.99.0-preview.3.231\tools\Xamarin.Android.Common.targets(1460,3): error XAGJS7001: System.NullReferenceException: Object reference not set to an instance of an object.
at Xamarin.Android.Manifest.ManifestDocumentElement.ResolveType(String type, ICustomAttributeProvider provider, IAssemblyResolver resolver)
at Android.App.ApplicationAttribute.<>c.<.cctor>b__114_7(ApplicationAttribute self, ICustomAttributeProvider p, IAssemblyResolver r, TypeDefinitionCache cache)
at Xamarin.Android.Manifest.ManifestDocumentElement`1.ToAttributeValue(String name, T value, ICustomAttributeProvider provider, IAssemblyResolver resolver, TypeDefinitionCache cache, Int32 targetSdkVersion)
at Xamarin.Android.Manifest.ManifestDocumentElement`1.ToAttribute(String name, T value, String packageName, ICustomAttributeProvider provider, IAssemblyResolver resolver, TypeDefinitionCache cache, Int32 targetSdkVersion)
at Xamarin.Android.Manifest.ManifestDocumentElement`1.<>c__DisplayClass8_0.<ToElement>b__1(String e)
at System.Linq.Enumerable.IteratorSelectIterator`2.MoveNext()
at System.Linq.Enumerable.IEnumerableWhereIterator`1.MoveNext()
at System.Xml.Linq.XContainer.AddContentSkipNotify(Object content)
at Xamarin.Android.Manifest.ManifestDocumentElement`1.ToElement(T value, ICollection`1 specified, String packageName, TypeDefinitionCache cache, ICustomAttributeProvider provider, IAssemblyResolver resolver, Int32 targetSdkVersion)
at Android.App.ApplicationAttribute.ToElement(IAssemblyResolver resolver, String packageName, TypeDefinitionCache cache)
at Xamarin.Android.Tasks.ManifestDocument.CreateApplicationElement(XElement manifest, String applicationClass, List`1 subclasses, TypeDefinitionCache cache)
at Xamarin.Android.Tasks.ManifestDocument.Merge(TaskLoggingHelper log, TypeDefinitionCache cache, List`1 subclasses, String applicationClass, Boolean embed, String bundledWearApplicationName, IEnumerable`1 mergedManifestDocuments)
at Xamarin.Android.Tasks.GenerateJavaStubs.MergeManifest(NativeCodeGenState codeGenState, Dictionary`2 userAssemblies)
at Xamarin.Android.Tasks.GenerateJavaStubs.Run(Boolean useMarshalMethods)
at Xamarin.Android.Tasks.GenerateJavaStubs.RunTask()
at Microsoft.Android.Build.Tasks.AndroidTask.Execute() in /Users/runner/work/1/s/xamarin-android/external/xamarin-android-tools/src/Microsoft.Android.Build.BaseTasks/AndroidTask.cs:line 25

These does not appear to be valid settings anyway, but I updated the code so it would throw a more appropriate ArgumentException instead of NullReferenceException in a random location inside the method.