Add usesCleartextTraffic to Tethering

usesCleartextTraffic needs to be true for the networkstack process so
that the NetworkStack module can use no-encrypted probes to detect
captive portals.

When loaded in the networkstack process, all packages in process must
set usesCleartextTraffic=true, otherwise there may be races causing
the flag not to be set for the process.

Bug: 161860610
Test: CtsTetheringTest, TetheringTests
Change-Id: Ife03ee0c7096ea242eb701b297a69b471e15b436

Tethering/AndroidManifestBase.xml

@@ -23,6 +23,7 @@
     <application
         android:label="Tethering"
         android:defaultToDeviceProtectedStorage="true"
-        android:directBootAware="true">
+        android:directBootAware="true"
+        android:usesCleartextTraffic="true">
     </application>
 </manifest>