[generator] Add [SupportedOSPlatform] to bound constant fields (#1038)

Context: #1037

Currently we do not add `[SupportedOSPlatform]` attributes to bound
`const` fields, which can lead to customers using them on platforms
where they are not supported:

	namespace Android.Telecom {
	    public partial class TelecomManager : Java.Lang.Object {
	        [Register("ACTION_POST_CALL", ApiSince=30)]
	        public const string ActionPostCall = "android.telecom.action.POST_CALL";
	    }
	}

Update `generator` so that `[SupportedOSPlatform]` is emitted for
such `const` fields:

	namespace Android.Telecom {
	    public partial class TelecomManager : Java.Lang.Object {
	        [Register("ACTION_POST_CALL", ApiSince=30)]
	        [global::System.Runtime.Versioning.SupportedOSPlatformAttribute ("android30.0")]
	        public const string ActionPostCall = "android.telecom.action.POST_CALL";
	    }
	}

Note this does not address the `enum` part of #1037, which will
require a much more involved fix.

Author: jpobst

tests/generator-Tests/Unit-Tests/CodeGeneratorTests.cs

@@ -736,6 +736,24 @@ public void SupportedOSPlatform ()
 
 			StringAssert.Contains ("[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain SupportedOSPlatform!");
 		}
+
+		[Test]
+		public void SupportedOSPlatformConstFields ()
+		{
+			// Ensure we write [SupportedOSPlatform] for const fields
+			var klass = new TestClass ("java.lang.Object", "com.mypackage.foo");
+			var field = new TestField ("java.lang.String", "bar").SetConstant ("MY_VALUE");
+
+			field.ApiAvailableSince = 30;
+
+			klass.Fields.Add (field);
+
+			generator.Context.ContextTypes.Push (klass);
+			generator.WriteType (klass, string.Empty, new GenerationInfo ("", "", "MyAssembly"));
+			generator.Context.ContextTypes.Pop ();
+
+			StringAssert.Contains ("[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain SupportedOSPlatform!");
+		}
 	}
 
 	[TestFixture]

tools/generator/SourceWriters/BoundField.cs

@@ -31,6 +31,7 @@ public BoundField (GenBase type, Field field, CodeGenerationOptions opt)
 			if (field.IsEnumified)
 				Attributes.Add (new GeneratedEnumAttr ());
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, field, opt);
 			SourceWriterExtensions.AddObsolete (Attributes, field.DeprecatedComment, opt, field.IsDeprecated, isError: field.IsDeprecatedError, deprecatedSince: field.DeprecatedSince);
 
 			if (field.Annotation.HasValue ())