Support custom offsets from SearchResultSet

Support custom offsets from SearchResultSet

Work to support interleaved AB testing of search will display
interleaved results of two search configurations on the first page of
results, but standard results on all pages other than the first page.
This means that while 20 results may be requested, the next 'new' result
of the primary query may be at position 10. Allow the SearchResultSet
to declare what the new offset is.

Bug: T150032
Change-Id: I14c0c33249fcdb66f72f5966e2aa72781a34daee