Define BundleSession for MediaMetrics

This brings the extensability of bundles in existing metrics out to
the level of an entire metric. Bundles allow us to present data to the
metrics module (moving to mainline) and statsd (already in mainline) so
that approved metrics can be plumbed through without an OTA system update.
All metrics data will be vetted within the metrics module and statsd
to ensure only approved metrics data leaves the device.

Bug: 214594702
Test: atest CtsMediaHostTestCases
Change-Id: I8dd1acbc0d6166b7448b7ca0f5e6ec9f4a24e471