20450 Revert “Refactor initial route code (#19684)” (cla: yes, platform-ios)

20468 Refactor initial route code take 2 (cla: yes, platform-ios)

20486 Remove the dummy rasterizer delegate now that flutter_runner is in tree, and cleanup ctor params (cla: yes, platform-android, platform-fuchsia, platform-ios, waiting for tree to go green)

20487 Only create raster_thread_merge when explicitly requested by the embedding platform (cla: yes, platform-ios, waiting for tree to go green)

20492 Enable iOS plumbing for network security and add tests (cla: yes, platform-ios)

20522 Fixed the accessibility ios unit tests by sending in mock view controllers instead of views. (cla: yes, platform-ios)