Prevent duplicate posts from entering pipeline

This CL compares new notifications having an existing key to their
corresponding media data in order to avoid the unnecessary reprocessing
of duplicate posts.

Flag: com.android.systemui.media_controls_posts_optimization
Bug: 358645640
Test: atest MediaDataProcessorTest
Test: atest MediaControlPanelTest
Test: atest LegacyMediaDataManagerTest

Change-Id: I81237a68156eb61c42a9c4863709958fba4576d0