The installation: its key, made on first use, and its ID. A key made
now (InstallationIdentity.createdThisLaunch) is a new installation, so
it gets a new ID even where one was stored: that ID belongs to the key
that was lost, and a backend refuses a known ID under another key. Calls
at once share one load, so that one key never gets two IDs.
Keeps the installation your backend answered for this key, which may
not be the ID proposed. Only this opaque ID is stored here; the private
key never leaves the platform's keystore.