link media.metrics native strings to api-controlled java strings

add comments to native media.metrics code pointing to how they
are (and aren't) to be kept in synch with their java counterparts
that are part of the public API (and therefore pretty much fixed).

Bug: 72492234
Test: compilation
Change-Id: Ic096545a5cdfb2160c6b66438d345d8e625384b9