Freeze netd AIDL interfaces as v4.

This adds BPF tethering offload APIs that are necessary to ensure
that BPF tethering respects data limits. Specifically:

 interface INetd {
   ...
+  android.net.TetherStatsParcel[] tetherOffloadGetStats();
+  void tetherOffloadSetInterfaceQuota(int ifIndex, long quotaBytes);
+  android.net.TetherStatsParcel tetherOffloadGetAndClearStats(int ifIndex);
   ...
 }

 parcelable TetherOffloadRuleParcel {
   ...
+  int pmtu = 1500;
 }

 parcelable TetherStatsParcel {
   ...
+  int ifIndex = 0;
 }

Generated via:
  m netd_aidl_interface-freeze-api
  git add server/aidl_api/netd_aidl_interface/4/

Test: m, builds, atest
Bug: 150736748
Change-Id: I0b0c67d43ddf2936df57fd1df8ededec51dc6189
Merged-In: I0b0c67d43ddf2936df57fd1df8ededec51dc6189