SurfaceFlinger: handle wallpaper scenario in Scheduler

Use the fact that a layer contains wallpaper in Scheduler's
algorithm when picking a refresh rate. Wallpaper should be presented
in the lowest active refresh rate to preserve power.

Test: systrace during wallpaper
Bug: 124065567
Change-Id: Ibb18e62eaf1a6b9b3ab1ec18a3135fb17c8f57fc