Make seccomp honor setenforce

Note: This code is in frameworks, since we might want to have tools
linked against seccomp_policy that continue to enforce seccomp

Bug: 35950093
Test: Builds, Seccomp: 2 is in zygote & zygote64 status
      Run setenforce 0 && stop && start
      Seccomp: 0 is in zygote & zygote64 status

Change-Id: I797a6c29f58725f84089c570f784125fb99f57ce