write
Stores the encryption key on TBS via PUT /v1/backup/encrypt-key.
This is used during the backup flow. It sends both the Authorization header (client API key) and the X-Firebase-Token header (Firebase ID token), along with the encryption key in the request body.
On a 401 response, the token is refreshed and the request is retried once.
Return
A confirmation string on success.
Parameters
content
The encryption key to store.
Throws
If the token is unavailable or the TBS request fails.