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. Add ExternalMediaMetadata to the external input source and state.
. Add ExternalMediaMetadataChangeObserver for the external pipeline to report media state change.
. Report external pipeline state to the Assistant Server via MediaParams.

Design doc: internal http://go/voice-control-for-external-audio-pipeline

Bug: internal b/80156336
Change-Id: I68ec11cc6fe4f368c34fe705c05a0f50ee9011cc
Reviewed-on: https://chromium-review.googlesource.com/1192116Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588177}