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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:b2bbc67f67dad09bbecd8fc3d37854d4684c1bf6)
Merged-In: I04f6c501835f8b761a8fc985f5862f896ea16b4d
Change-Id: I04f6c501835f8b761a8fc985f5862f896ea16b4d