servicemanager: selinux: set selinux callbacks early

SELinux library error logs are never be shown in logcat if
there's a failure before the callbacks are set. We missed
all logs from service context initialization because of this.

Bug: 33746484
Test: Restart new service manager to make sure context init logs
show up in logcat.

Change-Id: I5479cf5fe10ceb1a210eda26946b6ea344792e24
Signed-off-by: Sandeep Patil <sspatil@google.com>