Partition mediametrics serialization code

Separate files for parcel-based and bytestring-based serialization.
This simplfies moving mediametrics to a mainline module, where
we don't use the Parcel format.

Bug: 384759099
Flag: EXEMPT refactor
Test: atest android.media.metrics.cts.MediaMetricsAtomTests
Test: run in both variants: "in framework" and "in module"
Change-Id: I94ba1451a9b55825f52d208940b23da7a8eac526