Fix freeze rate calculations

Using the actual render duration to determine if the previous frame was
dropped is flawed at the beginning of playback because the first frame
never has a previous frame.

Bug: 234833109
Test: atest VideoRenderQualityTrackerTest#capturesFreezeRate
Change-Id: I6d3db5e662462ec802e368e5a611c0bbb2f463b3
3 files changed