Update behavior for incoming SMS path.

1) On KitKat, unencrypted SMS messages are never stored in
   TextSecure unless it is set as the system-wide default.

2) On KitKat, if TextSecure is set as the system-wide default,
   provide an option to change the default to a different app.

3) Don't store the TextSecure challenge on KitKat+ devices.
Better support for local storage with passphrase disabled.

1) Never get into a state where messages aren't displayed
   unencrypted.

2) Fix bug where locked DB required launching twice.
Use modern icons in info and alert dialogs for ICS+