[lib][native] Disambiguate URIs in media-cache

Summary:
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

Test Plan: 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

Reviewers: atul, ashoat

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8324