Implement sliding window flow helper

Given a Flow<T> and window duration, this converts the flow to a
Flow<List<T>> where the list is all the elements emitted by the source
flow within the specified window.

This can be used to smooth out signal data

Bug: 396460215
Test: atest FlowUtilTests
Flag: EXEMPT not used where yet
Change-Id: Iaa464a287032b0083cbed0ad21f872825406d1e2