20797 Flutter Engine Beta 1.21.0-9.2.pre cherry picks (cla: yes, platform-android, platform-ios)

20800 Add ability to disable the raster thread merger (cla: yes, platform-android)

20812 Revert “Pass platform configuration to Dart VM for insecure socket policy” (cla: yes, platform-android, platform-ios)

20818 Revert “Pass platform configuration to Dart VM for insecure socket po… (cla: yes, platform-android, platform-ios)

20829 Reland “Pass platform configuration to Dart VM for insecure socket policy” (cla: yes, platform-android, platform-ios)

20838 Flutter 1.20.3 engine cherrypicks (cla: yes, platform-android, platform-ios)