Move tethering related hidden API flag file entries to packages/modules/Connectivity/Tethering

Bug: 179354495
Test: m com.android.tethering
      - ensure that the generated APEX is byte-for-byte identical
        before and after these changes.
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - make sure that they are not changed by this.
Change-Id: I805f6f8553fb17f4c0aa78c3314786a8351ca1f4