SF: Fully release RefreshRateOverlay buffers on disable

Add a RefreshRateOverlay destructor to ensure that all GraphicBuffers used by the overlay are properly released when the overlay is disabled.

Bug: 365444202
Test: Enabled and disabled RefreshRateOverlay multiple times, verified no memory leaks in logcat (adb shell dumpsys SurfaceFlinger | grep RefreshRateOverlayBuffer).
Flag: EXEMPT bug fix
Change-Id: I7145107dbb86960c6ff7095487497d334388293e