Move NetworkStackTests to presubmit

The tests have been green in postsubmit for a while and run very
quickly.

Test: atest NetworkStackTests
Change-Id: I9dbbf9046dc0878c355a5ae791e31166038df796
diff --git a/packages/NetworkStack/TEST_MAPPING b/packages/NetworkStack/TEST_MAPPING
index 55ba591..fe9731fe 100644
--- a/packages/NetworkStack/TEST_MAPPING
+++ b/packages/NetworkStack/TEST_MAPPING
@@ -1,5 +1,5 @@
 {
-  "postsubmit": [
+  "presubmit": [
     {
       "name": "NetworkStackTests"
     }