Configure temp user engaged timeout.

Adds support to configure the duration media session remains in the engaged
state after active playback via command line flag. This allows for quicker
testing and development of media app behavior.

Flag: com.android.media.flags.enable_notifying_activity_manager_with_media_session_status_change
BUG: 281762171
Test: adb root; adb shell device_config put media media_session_temp_user_engaged_duration_ms 10000; adb shell dumpsys media_session

Change-Id: I7c249c2e8c241d9b391afe1b6fe201a301b46120
Signed-off-by: Pradeep Sawlani <sawlani@google.com>