before parsing message stanza for encryption parameters.
Otherwise we might not know what our own device-id/sid is, and therefore
can't decrypt the incoming message.
Fixes#2733
This wrongly stored value wasn't inlcuded in the published the bundle
because the libsignal store was used, which had the right value for the public key.
Instead, this value was used locally by being passed to the libsignal
session builder to verify signed prekey.