Add more logging to the Tethering SharedLog

This may at first seem a tad spammy, but a cursory examination suggests
most of this is the Tethering subsystem doing more work than necessary.
Some of this can be addressed with some simple changes in the near term.

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
    - watch "adb shell dumpsys connectivity tethering | tail 60"
      shows new log entries

Bug: 29337859
Bug: 32163131
Bug: 63334101
Change-Id: I6219e56d571854a2c61e1da63095e319e4bd3dc1