Expand gainmap screencapture:

* Add gainmap support to ScreenCapture proper
* Remove attachGainmap flag in screencap, since flag guarding is done
  entirely in SF by both the true_hdr_screenshots flag AND the seamless
  transition hint
* Plug in PNG gainmap encoding

Bug: 329465218
Flag: com.android.graphics.surfaceflinger.flags.true_hdr_screenshots
Test: Trigger screencapture

Change-Id: Id4fdb0c32d09f7a94a2acdf6c3cf2698861c8ec9