Clean up uses of refreshStartTime for stats.

refreshStartTime is keeps track of frame refreshes for
stats and can be set from SurfaceFlinger. This makes
subsequent changes in refactoring
SurfaceFlinger#composite cleaner and makes tracking the
metric more consistent between changes.

The refreshStartTime field can be removed entirely from
LayerFE's CompositionResult and removed as an argument
in LayerFE#onPreComposition.

Bug: b/294936197, b/329275965
Test: presubmit
Change-Id: I97988315f8982d05aec4b2684d4f5f1826a0fefa