Only create ProtoOutputStream when onGetData() is called.

The exception is EventMetricProducer. Each EventMetricProducer will still have a ProtoOutputStream
Because LogEvent comes as a fixed 4K, it's more memory efficient to have an 8k ProtoOutputStream for
storing the events.

Also removed finish() api in MetricProducer, which was intended to use with Dropbox.

Test: statsd_test & dogfood app
Bug: 70393808
Change-Id: I2efe4ecc76a88060a9aa5eb49d1fa6ea60bc5da8