Revert "mediacodec/mediaextractor: allow multiple Seccomp extensions"

Seeing the following build errors:

  $ . ./build/envsetup.sh  && lunch aosp_dragon-userdebug
  $ m -j 50
  ...
  Starting build with ninja
  ninja: Entering directory `.'
  ninja: error: 'device/google/dragon/seccomp/mediaextractor-seccomp.policy', needed by 'out/target/product/dragon/obj/ETC/mediaextractor-seccomp.policy_intermediates/mediaextractor-seccomp.policy', missing and no known rule to make it
  make: *** [ninja_wrapper] Error 1
  make: Leaving directory `/SDD/master'


This reverts commit 87a80ef1.

Change-Id: I5d039845c69b18f3a7199458c5301314d006f43b