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