Improve image captioner (#19024)
Follow up #18924
Fixes #19033
Fixes #19039
Summary of the issue:
AI Image Descriptions turned on causes NVDA to load more slowly to some extent
the image description is not shown in braille
Description of user facing changes:
Improved NVDA startup speed to some extent when AI image descriptions is enabled.
Show image description in braille
Reduced NVDA memory usage to some extent.
Description of developer facing changes:
Improve grammar and variable name
Show image description message in main thread
Load image descriptioner in background
