Make MMS more asynchronous and consistent with new SMS types.

1) We now delay MMS notifications until a payload is received,
   or there's an error downloading the payload.  This makes
   group messages more consistent.

2) All "text" parts of an MMS are combined into a second text
   record, which is stored in the MMS row directly rather than
   as a distinct part.  This allows for immediate text loading,
   which means there's no chance a ConversationItem will resize.

   To do this, we need to include MMS in the big DB migration
   that's already staged for this application update.  It's also
   an "application-level" migration, because we need the MasterSecret
   to do it.

3) On conversation display, all image-based parts now have their
   thumbnails loaded asynchronously.  This allows for smooth-scrolling.
   The thumbnails are also scaled more accurately.

src/org/thoughtcrime/securesms/mms/AudioSlide.java

@@ -27,6 +27,7 @@ import android.graphics.Bitmap;
 import android.graphics.BitmapFactory;
 import android.net.Uri;
 import android.provider.MediaStore.Audio;
+import android.widget.ImageView;
 
 public class AudioSlide extends Slide {
 
@@ -49,10 +50,10 @@ public class AudioSlide extends Slide {
   }
 	
   @Override
-    public Bitmap getThumbnail() {
+  public Bitmap getThumbnail(int maxWidth, int maxHeight) {
     return BitmapFactory.decodeResource(context.getResources(), R.drawable.ic_menu_add_sound);
   }
-	
+
   public static PduPart constructPartFromUri(Context context, Uri uri) throws IOException, MediaTooLargeException {
     PduPart part = new PduPart();