SF Generalization of Refresh Rates: Adding layer priority hint

Test: Device boots. Observe logs for errors.
Test: Open Swappy and Chrome in split screen, tap between apps,
      SF prints the correct priority.
Test: Open Chrome, play video. SF prints correct priority.
Test: SF prints the correct priority received.
Bug: 142507166
Change-Id: I5fa8a857c950db01f42a380a72d098039badc289