Recompute the mNextFrameStartUnstuffed on every frame

At runtime, the intended_vsync difference is not constant value of
16.66ms.  If JankTracker repeatedly adds 16.66ms to
mNextFrameStartUnstuffed, it might be difted away after the tiny
differences being accumulated over many frames.

This Cl re-computes mNextFrameStartUnstuffed based on intended_vsync
for each triple buffering frame.

Test: existing JankTrackerTests

Change-Id: Ifa0b456be61905648f87327bddc3328e9abfef0a