Fix external synthetic descriptor for api stubs

Change-Id: I368dbfcd575723adf89356ca0129881740758488
diff --git a/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java b/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java
index bedc228..fc487f7 100644
--- a/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java
+++ b/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java
@@ -58,7 +58,7 @@
     CONST_DYNAMIC("$Condy", 30, false),
     ENUM_CONVERSION("$EnumConversion", 31, false, true),
     API_MODEL_OUTLINE("ApiModelOutline", 32, true, false, false),
-    API_MODEL_STUB("", 33, false, true, true);
+    API_MODEL_STUB("ApiModelStub", 33, false, true, true);
 
     static {
       assert verifyNoOverlappingIds();