ItemsContentProvider: Move some code to MoreItemsJsonParser.

So it's simpler to create a test case for it.
Add a MoreItemsJsonParser test.
DecisionTree: Close the JsonReader explicitly in case it is necessary.
MoreItemsJsonParser: Use JsonReader instead of JsonTokener.

So we can parse the InputStream directly instead of copying its contents
out to a String first.
ItemsContentProvider: Use JsonReader instead of JsonTokener.

This lets us parse the stream directly instead of copying its contents
into a String first.
Also moved the parsing code into LoginUtils so it's easier to test it
separately from ItemsContentProvider.
Remove an unnecessary import.