Unify media.codec and media.extractor Minijail usage.

That way we can modify this code in one place to take a list of seccomp
policy files.

This CL removes around 70 lines by unifying identical code. No change
in behavior.

Bug: 34723744
Test: media.codec, media.extractor start, have Seccomp.
Test: cat /proc/`pgrep .codec`/status | grep Seccomp
      Seccomp:    2
Test: cat /proc/`pgrep .extractor`/status | grep Seccomp
      Seccomp:    2
Change-Id: Ia8be7b9c3d6163804d2a45954f8d03315ae32e7a