Statsd BroadcastSubscribers can receive 'cookies'

Subscribers can now provide additional Strings that will be passed back to
it when an anomaly is detected. This way, the Subscriber can be informed
about anomaly-specific information has meaning to it.
Statsd itself does not use these strings and doesn't care what they are
- it simply passes them back to the subscriber.

Change-Id: If269872397c4ed2c67e41d54d850cdc664d4f73e
Fixes: 74012329
Test: run cts-dev -m CtsStatsdHostTestCases -t android.cts.statsd.alert.BroadcastSubscriberTests