[IMAP] Fix migration to new Gmail authentication

Currently the settings would be migrated only if user opened the
Settings dialog. The GmailPasswordRequester expects that the server
settings is migrated correctly and always tries to authenticate via
XOAUTH2, even if the configuration says "PLAIN" or other mechanism.
This breaks login as we try to send XOAUTH2 tokens as PLAIN credentials.

This change ensures that the encryption and auth mechanism settings
is correctly migrated on resource startup.

CCBUG: 378857
CCBUG: 378876