Give up PGP decryption when intent is cancelled

When receiving a PGP message which is not encrypted with YOUR key,
OpenKeychain shows a dialog, which tells you the private key to decrypt
the message is unavailable. However, Conversations won't give up
decrypting the message. So whether the subsequent messages are
decryptable or not, the decryption is blocked at the current message.
The commit fixes the bug in this way: Give up the current message when
the decryption intent is cancelled, so that subsequent messages can be
handled.

this undos 8a729061. as it turns out 4.1
and 4.0 only break when checking if a cert is self signed.

The self signed certificates created by OpenFire (Not sure if other
certs are affected as well) will crash the Java/Android TLS stack when
accessing getSubjectAlternativeNames() on the the peer certificate.

This usually goes unnoticed in other applications since the
DefaultHostnameVerifier checkes the CN first. That however is a
violation of RFC6125 section 6.4.4 which requires us to check for the
existence of SAN first.

This commit adds a work around where in self signed certificates we
check for the CN first as well. (Avoiding the call to
getSubjectAlternativeNames())