Remove freezeIterable() when data items are processed immediately

In some samples, we were calling freezeIterable to make a
copy, and this is not necessary. We only need to make a
copy when a data item is used later on a different
thread, and comments are added for those situations.

Bug: 22007852
Change-Id: I78f03bdc5ce42d40e802527bbb57ec9848ccb68e