Add TetheringPrivilegedTests to postsubmit

Also ignore DadProxyTest before S because it is only enabled from S.

To using ignore test rule, replace AndroidJunit with DevSdkIgnoreRunner.
It is fine because the we don't use the test UI (e.g. running test in UI
thread).

Test: atest TetheringPrivilegedTests
Change-Id: I701ee877520f78ded2394adbae1f4d9dd612d700
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 54291a1..be0e040 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -22,6 +22,11 @@
       "name": "TetheringIntegrationTests"
     }
   ],
+  "postsubmit": [
+    {
+      "name": "TetheringPrivilegedTests"
+    }
+  ],
   "mainline-presubmit": [
     {
       "name": "CtsNetTestCasesLatestSdk[CaptivePortalLoginGoogle.apk+NetworkStackGoogle.apk+com.google.android.resolv.apex+com.google.android.tethering.apex]",