[async] Use ZX_TIMER_SLACK_LATE when creating async_loop_t's timer

Use ZX_TIMER_SLACK_LATE instead of "0u" to ensure the timer does not
fire early after we've enabled default slack via job policy (ZX-931).

Bug: ZX-3089 #comment use late slack
Test: /boot/test/sys/async-loop-test
Change-Id: I8fc7fb56ec34120512468f4aaffc52e5a9d0cad8
diff --git a/system/ulib/async-loop/loop.c b/system/ulib/async-loop/loop.c
index 83ff018..fe75712 100644
--- a/system/ulib/async-loop/loop.c
+++ b/system/ulib/async-loop/loop.c
@@ -165,7 +165,7 @@
 
     zx_status_t status = zx_port_create(0u, &loop->port);
     if (status == ZX_OK)
-        status = zx_timer_create(0u, ZX_CLOCK_MONOTONIC, &loop->timer);
+        status = zx_timer_create(ZX_TIMER_SLACK_LATE, ZX_CLOCK_MONOTONIC, &loop->timer);
     if (status == ZX_OK) {
         status = zx_object_wait_async(loop->timer, loop->port, KEY_CONTROL,
                                       ZX_TIMER_SIGNALED,