Modern send icons in coversations

Make the send-button appearance more similar to the default Android
messenger and Google Talk applications (that send-arrow).
Made a similar icon to indicate secure messages.

![ic_send_holo_light](https://f.cloud.github.com/assets/373823/226028/ff40fb02-8611-11e2-89a9-9c13e8f17ae8.png)
![ic_send_encrypted_holo_light](https://f.cloud.github.com/assets/373823/226029/00c690ae-8612-11e2-8f47-3fc4e9f024dc.png)

Also made them fade out and disable when the EditText is empty.