Add initial support for display readback

We've had readback support for testing purposes for many, many releases
now, but now fullscreen capture is becoming more common, so we want to
hook this up into production.

Readback currently must be negotiated on the main surfaceflinger thread,
so the implementation will schedule a composition request to piggyback
readback parameters on top of (this is zero overhead during steady-state
compositing), pass the readback buffer over to HWC prior to validate(),
then receive the readback fence after present().

This is flag guarded in two ways: an aconfig flag to control the
rollout wholesale, and also a device opt-in. The device opt-in is
necessary since readback on some devices may not be production-ready due
to its historical test-only status.

Also fix a minor issue where we always add an excludedLayerId set of {0}
due to aggregate initialization.

Bug: 384816457
Flag: com.android.graphics.surfaceflinger.flags.readback_screenshot
Test: builds
Test: adb screencap
Change-Id: Ic70d01140748d34880cc6cbe740685153bcf5b27