Expand logging capabilities of Tethering subsystem

Introduce a SharedLog that can be shared among all Tethering
components.  All log entries go to the same log, and each includes
its subsystem name.  Additionally, all entries emitted to the
system log shared the same tag, so can easily be watched.

More subsystems can inherit a SharedLog, but to minimize cherry-pick
conflicts we limit ourselves to these changes only.

Test: as follows
    - built
    - flashed
    - booted
    - log output observed via dumpsys
    - runtest frameworks-net passes
Bug: 32163131
Bug: 36988090
Bug: 38218697
Change-Id: I1229d4c55987402d84b0d4a2c8a595e0c829aa3e