[native] Rename holder prop to blobURI
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
Depends on D8322
Test Plan: Flow, also ran the native app, cleared media cache and ensured that encrypted multimedia are still displayed as expected
Reviewers: atul, ashoat
Reviewed By: atul
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D8323