Allow deletion of media through preview.

When viewing a media in the media preview, you can delete it by pressing
a delete button on the action bar. It will then ask you to confirm your
choice. If you confirm, it will delete the attachment from the database
and from disk. If it was the only attachment for that message, the
message itself will also be deleted.

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

@@ -17,8 +17,10 @@
 package org.thoughtcrime.securesms.database;
 
 import android.content.Context;
+import android.database.ContentObserver;
 import android.database.Cursor;
 import android.net.Uri;
+import android.support.annotation.NonNull;
 
 import org.thoughtcrime.securesms.database.helpers.SQLCipherOpenHelper;
 
@@ -29,6 +31,7 @@ public abstract class Database {
   protected static final String ID_WHERE              = "_id = ?";
   private   static final String CONVERSATION_URI      = "content://textsecure/thread/";
   private   static final String CONVERSATION_LIST_URI = "content://textsecure/conversation-list";
+  private   static final String ATTACHMENT_URI        = "content://textsecure/attachment/";
 
   protected       SQLCipherOpenHelper databaseHelper;
   protected final Context             context;
@@ -59,6 +62,16 @@ public abstract class Database {
     cursor.setNotificationUri(context.getContentResolver(), Uri.parse(CONVERSATION_LIST_URI));
   }
 
+  protected void registerAttachmentListeners(@NonNull ContentObserver observer) {
+    context.getContentResolver().registerContentObserver(Uri.parse(ATTACHMENT_URI),
+                                                         true,
+                                                         observer);
+  }
+
+  protected void notifyAttachmentListeners() {
+    context.getContentResolver().notifyChange(Uri.parse(ATTACHMENT_URI), null);
+  }
+
   public void reset(SQLCipherOpenHelper databaseHelper) {
     this.databaseHelper = databaseHelper;
   }