Media metrics APIs: changes based on API review

1. Added the IntDef annotation for PlaybackStateEvent#getState
2. Made event construtors private go/android-api-guidelines#parcelable-creator.
3. Improved the JavaDoc of time-related methods
4. Added the JavaDoc for TrackChangeEvent#getTrackType
5. Made MediaMetricsManager final

Fix: 183940704
Fix: 185121406

Test: atest android.media.metrics.cts
Change-Id: I9f1edc23b4e78e3ebee358945ce16539b83e1830