22367 Revert “Rasterizer is initialized with an external view embedder (#22348)” (cla: yes, platform-ios)

22372 Reland “Do not involve external_view_embedder in submit frame process if threads are not merged. #22275” (cla: yes, platform-ios, waiting for tree to go green)

22403 [PlatformViewsController] Clear root_views_ in Reset (cla: yes, platform-ios, waiting for tree to go green)

22406 PlatformViewsController always make sure the touch events are finished (cla: yes, platform-ios, waiting for tree to go green)

22449 [flutter_releases] Flutter 1.22.4 engine cherrypicks (cla: yes, platform-android, platform-ios)