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