zygote: initialize selinux app contexts before fork()

Initializing before zygote fork() results in only doing this once,
which appears to be the original intent of seapp_context_init().
However, since it was indirectly called after fork() it was
happening on during every app launch.

Bug: 129704390
Test: ps -AZ; verify that apps have correct context
Test: with "#define DEBUG 1" in libselinux to verify that
selinux_android_seapp_context_reload() is only called once
in zygote.

Change-Id: Ibd6f0dda065f35def99f39ad9d44829a6b60cdc9