Fix typo in recently changed extra_apex_payload_key flag

The intention was to make extra_apex_payload_key take a comma separted
list of keys. Iterating over a string will only iterate over each
characters individually. Must split by "," to iterate over keys.

Test: Run signing test locally using tradefed.sh
Bug: 239991438
Change-Id: I51bfed53a43b1b0fc1e33e0dc3d418a727e6ff15
diff --git a/tools/releasetools/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index dbe79ab..4f2b1bf 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -1381,7 +1381,7 @@
         OPTIONS.extra_apks[n] = key
     elif o == "--extra_apex_payload_key":
       apex_names, key = a.split("=")
-      for name in apex_names:
+      for name in apex_names.split(","):
         OPTIONS.extra_apex_payload_keys[name] = key
     elif o == "--skip_apks_with_path_prefix":
       # Check the prefix, which must be in all upper case.