Allow explicit blacklisting with maxTargetSdk=0.

This allows other annotation properties to be added to blacklisted APIs,
such as publicAlternatives. The semantics are identical to not having
any annotation on the API.

Bug: 136102585
Test: m, inspect out/soong/hiddenapi/hiddenapi-flags.csv
Change-Id: I2fbcb630a4a7719984f31d9531aca4f4b658db95
diff --git a/tools/class2greylist/src/com/android/class2greylist/Class2Greylist.java b/tools/class2greylist/src/com/android/class2greylist/Class2Greylist.java
index bfa8076..e7e9716 100644
--- a/tools/class2greylist/src/com/android/class2greylist/Class2Greylist.java
+++ b/tools/class2greylist/src/com/android/class2greylist/Class2Greylist.java
@@ -65,6 +65,7 @@
     static {
         Map<Integer, String> map = new HashMap<>();
         map.put(null, FLAG_GREYLIST);
+        map.put(0, FLAG_BLACKLIST);
         map.put(26, FLAG_GREYLIST_MAX_O);
         map.put(28, FLAG_GREYLIST_MAX_P);
         map.put(29, FLAG_GREYLIST_MAX_Q);