SurfaceFlinger: use duration instead of offset

This change is using the duration given to sf/app without
converting them back to the legacy offset. This allows us to get
the expected vsync associated with a wakeup event.

This change also required some refactoring:
 - Move the periodic callback interface out of DispSync and in
   to DispSyncSource
 - Translate legacy offsets to duration in case that duration
   is not specified in the build files
 - Add support to VSD to expose the deadline timestamp of when
   a frame needs to be ready by

Bug: 162888874
Test: SF unit tests
Test: examine systraces

Change-Id: I87a53cc7dea931d3c195eab6842e003ca4516885