SF: flush pending fences before resetting vsync model

This allows using the most recent vsync timestamp for prediction when HWVsync callabcks are enabled.

Bug: 407541438
Bug: 272802278
Test: manual
Flag: com.android.graphics.surfaceflinger.flags.reset_model_flushes_fence
Change-Id: I81035020bd543c8f054c89c77b72fd35b8df6be0