Move flaky HIDL VHAL impl test to post submit.

The tests is flaky by nature because it depends on timer. Although
the flakiness is low, it is too flaky to run as presubmit.

Test: atest -s emulator-5554 DefaultVhalImplTest
Bug: 288197123
Change-Id: I735e3e6197769e47372f025d7eb82d54159a8ff8
diff --git a/automotive/vehicle/2.0/default/TEST_MAPPING b/automotive/vehicle/2.0/default/TEST_MAPPING
index bb58700..63ea3d5 100644
--- a/automotive/vehicle/2.0/default/TEST_MAPPING
+++ b/automotive/vehicle/2.0/default/TEST_MAPPING
@@ -2,7 +2,9 @@
   "presubmit": [
     {
       "name": "android.hardware.automotive.vehicle@2.0-manager-unit-tests"
-    },
+    }
+  ],
+  "postsubmit": [
     {
       "name": "android.hardware.automotive.vehicle@2.0-default-impl-unit-tests"
     }