update pins to use the CA's certificates

AndroidPinning will always validate using the system's trust manager, so
there is no use in putting cacert or self-signed certs as pins,
AndroidPinning will always fail since the system's trust manager will fail.
Instead include the CAs' high level, long lived keys for the set of commonly
used servers.  Any XMPP server not signed by one of the pinned CAs will
trigger the MemorizingTrustManager's TOFU/POP prompt.
guarantee a unique XMPP Resource

Since XMPP Servers can direct traffic based on the XMPP Resource,
ChatSecure should ensure that each install has a unique XMPP Resource.
Otherwise, if someone has two instances of ChatSecure running on the same
account, the XMPP Server might always direct traffic to one but not the
other.  Since ChatSecure does not expose the XMPP Resource to the user, it
should be a random string that is then stored

refs #563 https://github.com/guardianproject/ChatSecureAndroid/pull/563
make sure a SessionID is created, even if LocalUser is null or empty

refs #564 https://github.com/guardianproject/ChatSecureAndroid/pull/564
remove SessionID.getSessionID(), it duplicates toString() and is misnamed

A method called getSessionID() should return a SessionID, not a String, and
toString() already provides a method for getting the String representation
of a SessionID.

refs #564 https://github.com/guardianproject/ChatSecureAndroid/pull/564