Hardened TLS cipher suites and versions

As Georg Lukas wrote in his blog post about how Android handles TLS
handshake (http://op-co.de/blog/posts/android_ssl_downgrade/), an
explicit order of cipher suites and TLS versions must be supplied to
avoid having the weak (presumably broken) RC4 cipher at the top of the
preference list.

This commit adds the list included in the blog post to every TLS socket
creation, including IMAP, POP3 and SMTP, see Wireshark screenshots done
during testing at http://vsza.hu/k9mail-tls-hardening/
filter enabled cipher suites by availability
extracted method filterBySupport
filter TLS protocol versions by support as well
Extract WebDavSocketFactory
Refactor TrustedSocketFactory
Merge branch 'tls-hardening'