SF: Skip follower display presentation based on backpressure

Currently, SurfaceFlinger does not take follower display backpressure
into consideration when committing/presenting. This leads to the entire
system effectively latching on to the slowest follower refresh rate due
to backpressure in HWC if the refresh rate of the follower is lower
than the pacesetter.

This CL provides follower backpressure detection by checking for the
number of pending present fences for the display at the beginning of
Scheduler::onFrameSignal(). If there's a pending present, the scheduler
skips adding the display's frame targeter for commit/composite such
that the SF main thread and HWC does not lock up. The one exception is
when there's one pending present and the vsync is anticipated to happen
before the next pacesetter vsync, then we tolerate the temporary lockup
as it will resolve itself before the next pacesetter present. This
exception is critical in supporting refresh rates that are close to the
pacesetter's rates.

Please refer to go/pdarr-1pager for more.

Flag: com.android.graphics.surfaceflinger.flags.follower_display_backpressure
Bug: 389983019
Test: new scheduler and frame targeter unit tests && manual testing
Change-Id: Ie3cbe214410f0a15e40046fcfa0a2165814579e6