[Mono.Android] Enable NRT generator flag.

Author: jpobst

external/Java.Interop

@@ -107,7 +107,8 @@
       <_Api>$(IntermediateOutputPath)mcw\api.xml</_Api>
       <_Dirs>--enumdir=$(IntermediateOutputPath)mcw</_Dirs>
       <_FullIntermediateOutputPath>$([System.IO.Path]::GetFullPath('$(IntermediateOutputPath)'))</_FullIntermediateOutputPath>
-      <_LangFeatures Condition="$(AndroidApiLevel) &gt;= 30">--lang-features=default-interface-methods,nested-interface-types,interface-constants</_LangFeatures>
+      <_LangFeatures>--lang-features=nullable-reference-types</_LangFeatures>
+      <_LangFeatures Condition="$(AndroidApiLevel) &gt;= 30">$(_LangFeatures),default-interface-methods,nested-interface-types,interface-constants</_LangFeatures>
     </PropertyGroup>
     <Exec
         Command="$(ManagedRuntime) $(ManagedRuntimeArgs) $(Generator) $(_GenFlags) $(_ApiLevel) $(_Out) $(_Codegen) $(_Fixup) $(_Enums1) $(_Enums2) $(_Versions) $(_Annotations) $(_Assembly) $(_TypeMap) $(_LangFeatures) $(_Dirs) $(_Api)"