Makes sure to set up the mediaextractor x86_64 seccomp policy.

Three fixes
(1) Sets LOCAL_REQUIRED_MODULES_x86_64.
  Otherwise the policy file won't be in the system image at all.

(2) Drops the executable bit (chmod a-x) of the policy file.

(3) Incorporates the changes done on x86.policy between O and P.
  The previous patch seems to be a cherry-pick from AOSP, which
  does not take the P changes into account. (Hence mediaextractor
  just crashes if the previous policy file is applied to P.)

Bug: 79158930
Bug: 69073312
Test: ProcessMustUseSeccompTest#testMediaextractorHasSeccompFilter
Test: CtsMediaTestCases runs without mediaextractor/minijail error
Change-Id: Iada4c75883de03b5a44a06df33de428c2ceef3d5