Remove --json option from dumpsys media.audio_flinger

The more recent work that this was supposed to become is reflected in
dumpsys media.log, so this commit removes some code clutter that is not
used anymore.

Other Changes:
- Use std::make_unique for creating unique_ptr objects

Test: build, dumpsys media.audio_flinger
Bug: 68148948
Change-Id: I4841e118c2cdee4ada4c6cdc0fff1ec2e12b38c4