Flag: Trunk-stable flag for migrating pacesetter to display with highest refresh rate

PDARR is comprised of three main changes:
1. "Greedy" follower display presentation via follower display backpressure
2. Migration of pacesetter display to the highest refresh rate display
3. Proper buffer release/transaction callback if a buffer is used across multiple displays

This CL introduces a flag for #2, pacesetter display migration

Refer to go/pdarr-1pager for details.

Flag: com.android.graphics.surfaceflinger.flags.pacesetter_selection
Bug: 374072507
Test: N/A
Change-Id: I4417c415bd20c2c09ed1c5f3cf75b8d0e5e522ec