[web][native] Rename decryptMedia to fetchAndDecryptMedia

Summary: I think this name is more accurate to what this function does. Especially in context of why we're going to pass CSAT into it (next diffs)

Test Plan: Flow

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10907