Add the impl for the ability to query non-public Slices

Apps get Settings Slices through onGetSliceDescendants(), so adding some
codes here to make us be capable returning non-public Slices. As these
SliceData come from slice_index.db, where SliceDatabaseAccessor is the
middleman for us to access those data, so adding a parameter in
getSliceUris() to determine what data should be returned.

Bug: 141088937
Test: robotests
Change-Id: I411eb1ff194b7c8915b9e7309c684046dbde29fb