[lib] Add util functions to get blob URI

Summary:
Part of ENG-3966.

Introduced simple utility functions that should be used always when we want to get blob URI from encrypted multimedia, instead of accessing blobURI / holder directly.
These functions will be used in the next diff to make this one small and clean

Depends on D8371

Test Plan:
This stack is tested altogether, CI will fail at this point due to Flow breaking changes.
Actual test plan in the last diff.

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8372