slight refactoring of libmediametrics (client side)

-- isolate the pieces that know about the service so that clients never
   include anything service-related.
-- separate the marshalling / serialization code into a separate file to
   simplify working with both the SDK and the NDK.

Bug: 384759099
Flag: EXEMPT refactor
Test: atest mediametrics_tests
Test: atest CtsNativeMediaMetricsTestCases
Change-Id: I5a0c076380a3e8b4f4a6f827dd20be7ba78716e7