[XABT] Make NRT opt-out instead of opt-in. (#10093)

Help ensure we do not unintentionally add any additional non-NRT aware code to `Xamarin.Android.Build.Tasks` by switching `$(Nullable)` from `annotation` to `enable`.  This will subject any new files to NRT analysis by default unless they opt out with `#nullable disable`.

To switch to this mode:

- Add `#nullable disable` to any existing code files that have not been converted and currently have NRT warnings. (~200 files)
- Remove `#nullable enable` from any existing code files that have already been converted. (~50 files)

Author: jpobst

src-ThirdParty/Mono.Security.Cryptography/CryptoConvert.cs

@@ -29,6 +29,8 @@
 // Original source can be found at
 // https://github.com/mono/mono/blob/e2c5f4b0ad1a6b21ca0735f0b35b8611d4ad87b3/mcs/class/Mono.Security/Mono.Security.Cryptography/CryptoConvert.cs#L4
 
+#nullable disable
+
 using System;
 using System.Security.Cryptography;
 

src-ThirdParty/android-platform-tools-base/SymbolWriter.cs

@@ -1,3 +1,5 @@
+#nullable disable
+
 using Microsoft.Build.Framework;
 using System;
 using System.Collections.Generic;

src/Mono.Android/Android.App/IntentFilterAttribute.Partial.cs

@@ -1,3 +1,5 @@
+#nullable disable
+
 using System;
 
 namespace Android.App;

src/Xamarin.Android.Build.Tasks/Linker/External/Linker.Steps/BaseStep.cs

@@ -26,6 +26,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable disable
+
 using Mono.Cecil;
 
 namespace Mono.Linker.Steps {

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/Annotations.cs

@@ -26,6 +26,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable disable
+
 using System;
 using System.Collections.Generic;
 using Mono.Cecil;

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/AssemblyResolver.cs

@@ -26,6 +26,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable disable
+
 using System;
 using System.Collections.Generic;
 using System.IO;

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/BaseMarkHandler.cs

@@ -1,3 +1,5 @@
+#nullable disable
+
 using Mono.Cecil;
 using Mono.Linker.Steps;
 

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/LinkContext.cs

@@ -26,6 +26,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable disable
+
 using System;
 using System.Collections.Generic;
 using System.IO;

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/OverrideInformation.cs

@@ -1,3 +1,5 @@
+#nullable disable
+
 using System.Diagnostics;
 using Mono.Cecil;
 

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/Tracer.cs

@@ -26,6 +26,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable disable
+
 using System;
 using System.Collections.Generic;