metricsd: Cleanup on TERM signal.

metricsd should shutdown cleanly when receiving a TERM signal:
* stop the binder watcher.
* wait for the thread to quit.
* Exit cleanly.

Note: This is not directly used as we don't send SIGTERM on shutdown or
when stopping services yet.

Bug: 25670584
Change-Id: I878d1e67474c72d24790f3540470e37a23112a95