Fix typo in loading thumbnail

Currently there is a typo in thumbnail loader where we always
try to load the thumbnail regardless if we already loaded it
or not.

It's not the cause of the loop described in the bug but it's
still incorrect.

b/17365401

Change-Id: If6587f001d0e8dec81a71cf0e2ab6d298c187fbf
diff --git a/src/com/android/mail/ui/ThumbnailLoadTask.java b/src/com/android/mail/ui/ThumbnailLoadTask.java
index 71e2cc2..bfe6ae9 100644
--- a/src/com/android/mail/ui/ThumbnailLoadTask.java
+++ b/src/com/android/mail/ui/ThumbnailLoadTask.java
@@ -60,7 +60,7 @@
 
         final Uri thumbnailUri = attachment.thumbnailUri;
         final Uri contentUri = attachment.contentUri;
-        final Uri uri = (prevAttachment == null) ? null : prevAttachment.getIdentifierUri();
+        final Uri uri = attachment.getIdentifierUri();
         final Uri prevUri = (prevAttachment == null) ? null : prevAttachment.getIdentifierUri();
         // begin loading a thumbnail if this is an image and either the thumbnail or the original
         // content is ready (and different from any existing image)