Use SSL connection for T-Mobile.

The connection to the IMAP server cannot be made over a non-encrypted
cellular data connection. Instead, use the SSL connection to connect to
the IMAP server for T-Mobile which bipasses the network request. This
includes the following changes:
- Factor out syncing logic from OmtpVvmSyncService into a separate
  method so it can be called both directly and from the network callback
- Factor out voicemail fetching logic from FetchVoicemailReceiver into a
  separate method so it can be called both directly and from the network
  callback
- Use the SSL flag and port 993 for just T-Mobile queries.

Bug: 22802280
Change-Id: I938f4c7475327f9a133b74c0c9459edeece1a5b0
3 files changed