Add logging to statsd for job constraint changes.

Bug: 117846754
Bug: 111423978
Bug: 120941744
Test: `make statsd_testdrive &&
./out/host/linux-x86/bin/statsd_testdrive 150` and check output

Change-Id: Iadfdb07171b8d4f99b8f57008cc7c1fc2865bb8e