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