Refactor "parts" to contain MMS/PDU madness to MMS code paths.

Closes #4248
// FREEBIE

src/org/thoughtcrime/securesms/attachments/UriAttachment.java

@@ -0,0 +1,42 @@
+package org.thoughtcrime.securesms.attachments;
+
+import android.content.Context;
+import android.net.Uri;
+import android.support.annotation.NonNull;
+
+import org.thoughtcrime.securesms.crypto.MasterSecret;
+import org.thoughtcrime.securesms.util.MediaUtil;
+import org.whispersystems.libaxolotl.util.guava.Optional;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+public class UriAttachment extends Attachment {
+
+  private final Uri dataUri;
+  private final Uri thumbnailUri;
+
+  public UriAttachment(Uri uri, String contentType, int transferState, long size) {
+    this(uri, uri, contentType, transferState, size);
+  }
+
+  public UriAttachment(Uri dataUri, Uri thumbnailUri,
+                       String contentType, int transferState, long size)
+  {
+    super(contentType, transferState, size, null, null, null);
+    this.dataUri      = dataUri;
+    this.thumbnailUri = thumbnailUri;
+  }
+
+  @Override
+  @NonNull
+  public Uri getDataUri() {
+    return dataUri;
+  }
+
+  @Override
+  @NonNull
+  public Uri getThumbnailUri() {
+    return thumbnailUri;
+  }
+}