Dump counters in "dumpsys tethering bpf".

This CL adds the counters collected by the tethering programs to
the output of "dumpsys tethering bpf". The counter names are
fetched by JNI code at startup and then read by the debugging
code on demand.

Example from my device:

  Forwarding errors:
    TCP_CONTROL_PACKET: 67
    NON_GLOBAL_SRC: 13
    NON_GLOBAL_DST: 17
    LOCAL_SRC_DST: 7

Test: manual (debug-only code)
Change-Id: I7ec7992612556a4bca5f1b6bb52c655d6d870201
diff --git a/Tethering/jni/onload.cpp b/Tethering/jni/onload.cpp
index 3766de9..e31da60 100644
--- a/Tethering/jni/onload.cpp
+++ b/Tethering/jni/onload.cpp
@@ -24,6 +24,7 @@
 
 int register_android_net_util_TetheringUtils(JNIEnv* env);
 int register_com_android_networkstack_tethering_BpfMap(JNIEnv* env);
+int register_com_android_networkstack_tethering_BpfCoordinator(JNIEnv* env);
 
 extern "C" jint JNI_OnLoad(JavaVM* vm, void*) {
     JNIEnv *env;
@@ -36,6 +37,8 @@
 
     if (register_com_android_networkstack_tethering_BpfMap(env) < 0) return JNI_ERR;
 
+    if (register_com_android_networkstack_tethering_BpfCoordinator(env) < 0) return JNI_ERR;
+
     return JNI_VERSION_1_6;
 }