- Add a new volume_changed event, reported at the stream level.
- Only include changes to base streams (no aliases).
- Include the caller for each change. A caller is either:
- a pkg name (for external calls or known media sessions)
- a system server class's log tag (for internal calls,
disambiguates "android")
Bug: 19599935
Change-Id: Ia61b68ff1e7e2907a24972790ec052bfe099e665