releasetools: Allow skipping PRESIGNED APEXes.

This CL adds support that allows treating an APEX as pre-signed. We can
skip signing an APEX with `-e <apex-name>=` and
`--extra_apex_payload_key <apex-name>=`. Note that the payload_key and
container_key must be in consistent state - either they're both
PRESIGNED or none of them is. CheckApkAndApexKeysAvailable() has been
updated to perform the sanity check.

Bug: 123716522
Test: Run sign_target_files_apks.py with the above flags.
Test: python -m unittest test_sign_target_files_apks
Change-Id: Id1e2f3f2facd4a97a385983cc9b78c028f7e7e73