SurfaceFlinger: store fps instead of duration in LayerInfo

To get a better average of the content fps, we switch to store the
momentarily fps rate instead of the refresh duration.
For example:

Frame#0 at 0ms
Frame#1 at 100ms
Frame#2 at 111ms
Frame#3 at 122ms
Average based on duration is AVERAGE(100, 11, 11) = 40.6ms (25fps)
Average based on fps is AVERAGE(10, 90, 90) = 63fps

Test: app launch
Bug: 136558136
Change-Id: Icab848dd1f312498590f9735b8881ecdf0d24113