IssueTracker

Update getRefreshKey in TestPagingSource to handle refresh loads in the middle of the list.

Currently getRefeshKey simply returns anchorPosition. This can result in incorrect refresh key when the first generation has an initialKey set.

For example, initialKey = 50, initiaLoadSize = 5. First initial load returns items [50, 51, 52, 53, 54]. If pagingSource gets invalidated after a differ.getItem(4) call, the new initial load will return [4, 5, 6, 7, 8].