We used to require explicitly setting both (e.g. `-e foo.apex=` and
`--extra_apex_payload_key foo.apex=` to skip signing `foo.apex`).
This CL allows specifying `-e` alone to achieve the same result.
However, if a conflicting `--extra_apex_payload_key` is also specified,
that would be considered as a config error.
Bug: 131153746
Test: Run sign_target_files_apks.py with `-e foo.apex=` alone to skip
signing foo.apex.
Test: Run sign_target_files_apks.py with `-e foo.apex=` and
`--extra_apex_payload_key foo.apex=key` and expect assertion error.
Change-Id: Ia747f59ee726b60bdb1445024e749320171064c2
f98fa1094e