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

Closes #4248
// FREEBIE

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

@@ -0,0 +1,50 @@
+package org.thoughtcrime.securesms.attachments;
+
+import org.thoughtcrime.securesms.util.Util;
+
+public class AttachmentId {
+
+  private final long rowId;
+  private final long uniqueId;
+
+  public AttachmentId(long rowId, long uniqueId) {
+    this.rowId    = rowId;
+    this.uniqueId = uniqueId;
+  }
+
+  public long getRowId() {
+    return rowId;
+  }
+
+  public long getUniqueId() {
+    return uniqueId;
+  }
+
+  public String[] toStrings() {
+    return new String[] {String.valueOf(rowId), String.valueOf(uniqueId)};
+  }
+
+  public String toString() {
+    return "(row id: " + rowId + ", unique ID: " + uniqueId + ")";
+  }
+
+  public boolean isValid() {
+    return rowId >= 0 && uniqueId >= 0;
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) return true;
+    if (o == null || getClass() != o.getClass()) return false;
+
+    AttachmentId attachmentId = (AttachmentId)o;
+
+    if (rowId != attachmentId.rowId) return false;
+    return uniqueId == attachmentId.uniqueId;
+  }
+
+  @Override
+  public int hashCode() {
+    return Util.hashCode(rowId, uniqueId);
+  }
+}