Fix crash when running androidmk against frameworks/base/Android.mk

The was a variable on the right-hand side of the assignment to
LOCAL_MODULE_CLASS , which wants a non-nil scope

Bug: 64266643
Test: androidmk frameworks/base/Android.mk
Test: androidmk prebuilts/misc/common/ddmlib/Android.mk

Change-Id: I52d33f5e5cb1179f84d4df149ef804268d67f7fb

androidmk/cmd/androidmk/android.go

@@ -449,7 +449,7 @@ func sanitize(sub string) func(ctx variableAssignmentContext) error {
 }
 
 func prebuiltClass(ctx variableAssignmentContext) error {
-	class := ctx.mkvalue.Value(nil)
+	class := ctx.mkvalue.Value(ctx.file.scope)
 	if v, ok := prebuiltTypes[class]; ok {
 		ctx.file.scope.Set("BUILD_PREBUILT", v)
 	} else {