define media.metrics events for statsd

define atoms/messages that express the media.metrics events as they get
passed to statsd. This mirrors the data that was already being
collected via 'dumpsys media.metrics'.

Update the test app's list of acceptable uid's from which it takes
submissions to include the media.metrics service.

Bug: 118782504
Test: statsd/test_drive
Change-Id: I825f6cb7a9baa61f51c45b1b56a20dd94ae48015