SF: Fix FrameTimeline based FrameStats test

This doesn't fix real world usages (which were fine). It only fixes a
bug exercised by a corner case in a unit test.

Bug: 382769837
Test: LayerTypeTransactionTests/LayerTypeTransactionTest.RefreshRateIsInitialized
Flag: com.android.graphics.surfaceflinger.flags.deprecate_frame_tracker
Change-Id: I145bebf6cdaac2e89d2f76e5bb8e1d2c21cdff81