probeAccountNameHash
Hashes the sorted set of visible account names via AccountManager.
Privacy: high — account names (often email addresses) are directly identifying, so they are SHA-256 hashed rather than exposed. Visible accounts depend on the app's authenticator access; on modern Android most accounts are hidden.
Return
SHA-256 hex of the account names, or null if none are visible.
Parameters
context
Context used to obtain AccountManager.