Terminate statsd on a few signals including SIGTERM

- statsd is handling SIGHUP/SIGQUIT/SIGINT/SIGTERM, but doesn't exit.
- The expected behaviour is that statsd would exit after finalizing log
processing when those signals are received.

Bug: 139817664
Bug: 138764189
Test: Send SIGTERM to statsd and check if it is terminated. (note that
init will revive statsd)

Change-Id: Ie45821613664bf030754f835f5481c986ed3cede
Merged-In: Id1146d772f8c68892256ee7a3eea70837fee5c7a