TetherController: Process tether stats from bpf maps

Test: Manual test
1. Add two interface tether stats manually
2. Call getTetherStats() and check the content

Change-Id: I5bf7e57e67cf61829ef71fd7caccdb60d4c35213
diff --git a/server/TetherController.h b/server/TetherController.h
index 7bf8d32..7835cf6 100644
--- a/server/TetherController.h
+++ b/server/TetherController.h
@@ -70,8 +70,10 @@
         int sendAllState(int daemonFd) const;
     } mDnsmasqState{};
 
+    // BPF maps, initialized by maybeInitMaps.
     bpf::BpfMap<TetherIngressKey, TetherIngressValue> mBpfIngressMap;
     bpf::BpfMap<uint32_t, TetherStatsValue> mBpfStatsMap;
+    bpf::BpfMap<uint32_t, IfaceValue> mIfaceIndexNameMap;
 
   public:
     TetherController();
@@ -183,6 +185,7 @@
     int setForwardRules(bool set, const char *intIface, const char *extIface);
     int setTetherCountingRules(bool add, const char *intIface, const char *extIface);
 
+    void maybeInitMaps();
     void maybeStartBpf(const char* extIface);
     void maybeStopBpf(const char* extIface);