Integrate API Council feedback into Media Metrics APIs

Integrates the API Council's feedback into the Media Metrics
related APIs.
-- return types for the getMetrics() calls
-- definition of keys in the set of data that comes back

Bug: 35811992
Test: booted, CTS subset, checked dumpsys media.metrics output
Change-Id: Ib750f9631bd3453be3132760273ccfff0a891f88