Add some flags for readback

* Aconfig flag for the rollout
* Debug sysprop for configuring a device, since some devices won't be
  able to productionize readback at all.

Bug: 384816457
Flag: com.android.graphics.surfaceflinger.flags.readback_screenshot
Test: builds
Change-Id: I41ff3fe3d00dc93cc16ef8d1bc9e9ad1a392dcde