20647 fuchsia: Convert legacy permutations to build flag (affects: engine, affects: tests, cla: yes, code health, platform-fuchsia)

20681 fuchsia: Fix elevation issues (affects: engine, cla: yes, platform-fuchsia)

20697 fuchsia: Disable flaky test (affects: tests, cla: yes, platform-fuchsia)

20699 fuchsia: Rollback broken SDK and re-enable test (affects: tests, cla: yes, platform-fuchsia)

20704 fuchsia: Clean shell_unittests under FEMU (affects: tests, cla: yes, platform-fuchsia)

20761 fuchsia: Call out legacy rasterizer code (affects: engine, cla: yes, platform-fuchsia)