Migrate images.txt to json format

[yzqzss]

The media metadata storage size will increase, but the json key-value format is more readable and self-describing. Compared to the space used by the media files, the metadata is negligible.

• Code changes
• Compatible with previous images.txt
• Unit tests
• Documented

[TripleCamera]

Is it possible to store them in a database? Text files are bad at storing large volume of complex data.