releasetools: Fix an issue in handling PRESIGNED APEX keys.

For PRESIGNED APEXes, we should keep carrying the matching public keys
at /system/etc/security/apex.

Bug: 129148142
Test: Run sign_target_files_apks.py on a target_files.zip with presigned
      APEXes. Check the output zip.
Change-Id: I2e941fd9b10e99d2db9df1e5308cbbe8c760177b