ITS: centralize convert_to_y functions

Files affected:
  tests/scene7/test_multi_camera_switch.py
  tests/scene8/test_ae_awb_regions.py
  tests/scene_flash/test_auto_flash.py
  tests/scene_flash/test_preview_min_frame_rate.py
  utils/opencv_processing_utils.py

Other changes:
  test_ae_awb_regions.py --> change to RGB conversion from BGR

Bug: 333901109, 330382627
Testing: ran on test_multi_camera_switch.py
Change-Id: I6ac2cc870c5c5e180ea4ab8c5550ac18b34f916e
5 files changed