When building dispsync model, only callback on observed vsync + offset.

Only when hw vsyncs are turned off should we attempt to predict when
vsyncs should be. Otherwise, if we are in 90hz mode, and observe 16.6ms
vsyncs (60hz) because the display driver detected inactivity and dropped
down to 60hz, then we might dispatch early callbacks. This change
prevents those early callbacks from being dispatched.

Bug: 130684082
Test: systrace
Change-Id: I02bfd22beb00ddbe344c29722581217ad00a96af