Address NPE exception occuring in slidingFlow helper

Since we never add null values to the buffer, it looks like this might
be happening due to concurrency issues - since the LinkedList buffer
isn't thread-safe and we may be accessing it across threads if the flow
is collected using a multi-threaded dispatcher.

Bug: 410013484
Test: atest SystemUITests:SlidingWindowFlowTest
Flag: EXEMPT bugfix
Change-Id: Ic696d30a3ea351d475f4dfe8e41d2135524003cb