Mark "disable_synthetic_vsync_for_performance" flag as exported

The flag needs to be exported to be read in google3 apps like Android Auto Projected in order to be detect devices that have the flag enabled and adjust behaviour accoridngly.

Bug: 406147398

Flag: com.android.graphics.surfaceflinger.flags.disable_synthetic_vsync_for_performance

Change-Id: I04f6c501835f8b761a8fc985f5862f896ea16b4d