Flag: Trunk-stable flag to force GPU composition of slower folower displays

This CL introduces a new flag force_slower_follower_gpu_composition,
which forces GPU composition of follower displays whose configured
refresh rates are slower than the pacesetter.

Refer to go/pdarr-buffer for details.

Flag: com.android.graphics.surfaceflinger.flags.force_slower_follower_gpu_composition
Bug: 389982803
Test: N/A
Change-Id: I8dbecd87c399e709d985e5e1bf8da2da24353f9b