Add sticker support.

No sticker packs are available for use yet, but we now have the
latent ability to send and receive.

src/org/thoughtcrime/securesms/database/ObservableContent.java

@@ -0,0 +1,11 @@
+package org.thoughtcrime.securesms.database;
+
+import android.database.ContentObserver;
+import android.support.annotation.NonNull;
+
+import java.io.Closeable;
+
+public interface ObservableContent extends Closeable {
+  void registerContentObserver(@NonNull ContentObserver observer);
+  void unregisterContentObserver(@NonNull ContentObserver observer);
+}