This allows us to more easily create different dumps derived from the
same assets.

This moves media/source files from `assets/` to `assets/media/` and
dump files from `assets/` to `assets/extractordumps/` and
`assets/audiosinkdumps/` as appropriate. I intend to add
`assets/playbackdumps/` in a future CL.

PiperOrigin-RevId: 326986283