Part of ENG-3975.
Now the holder isn't accurate, I decided to name this value blobURI because it is of format: either https://keyserver/uploads/... or blob-service://blob-hash.

For media cache, we have two kind of URIs: blobURI - a cache key, and localURI/mediaURI which is a URI to local multimedia file. I needed to rename these accordingly

In short:

• holder -> blobURI
• uri -> mediaURI
• Added some more comments

Depends on D8323

Flow, also ran the native app, cleared media cache and ensured that encrypted multimedia are still displayed as expected. Also did some debugging to check if they're loading from cache