Move zygote's seccomp setup to post-fork

Before this change, seccomp filter setup is as early as in zygote's main
function.  To make it possible to split app and system server's filter,
this postpone the setup to after fork.  It also starts to call app
specific and system server specific setup function.

Test: (cts) -m CtsSecurityTestCases -t android.security.cts.SeccompTest
Bug: 63944145

Change-Id: I748735b478405098beac1e200d911c13ea60e380