Fix image cache bugs.

1) Index SlideDeckListener cache by MMS (id, timestamp) tuple.

2) Index parts by (id, content_id) tuples.

Fixes #840
Closes #3183
// FREEBIE

src/ws/com/google/android/mms/pdu/PduPart.java

@@ -142,6 +142,7 @@ public class PduPart {
       */
      public PduPart() {
          mPartHeader = new HashMap<Integer, Object>();
+         setContentId(String.valueOf(System.currentTimeMillis()).getBytes());
      }
 
      public void setEncrypted(boolean isEncrypted) {