Modified automatic decryption from clipboard - encrypted text is now hidden

It's a reference to https://github.com/openpgp-keychain/openpgp-keychain/issues/428. I've added another member field in `DecryptActivity` class called `mMessageText`, that stores encrypted text from clipboard.

Much more elegant solution would be to temporarily disable refreshing `mMessage` `EditText` by GUI update thread, but I failed to do it and I couldn't find any help online. Any hints would be much appreciated.

Greg