Re-enable modernized version of "optional" SSL/TLS

* Add "Accept all certificates" modes to incoming/outgoing secure choices
* Change URI scheme slightly to make "trust" a flag, not part of the
* Change Stores to know about new URI scheme
* Slightly rework Transport API to make "trust" an independent flag
* Adapt HostAuth to handle new Uri scheme
* Remove the old ambiguous "optional" code, which was allowing
    some unsigned certificates, but was *also* allowing TLS to
    optionally start (though not SSL, despite the UI strings.)
* Add a few unit tests to EmailContent
* Add logging and a bunch of comments to TrustManagerFactory, and a bit
    of simple cleanup to make it more readable.
* Add missing conversion of SSLException->CertificateValidationException
    in TLS so we get the correct certificate errors from TLS too.
* Re-enable TLS for accounts (which had a certificate problem)

Fixes bug http://b/2119755, http://b/1374780, and probably a raft of
earlier and/or external bugs about certificate problems.

Change-Id: Iaf99a8da3eaadaa4cdeec224737838b5d6813e55
1 file changed