Refactor creation of ConfiguredJarList from list of pairs

This change:
* Changes splitConfiguredJarPair to return an error rather than
  reporting it through the context.
* Extracts the splitting of a list of pairs into a pair of lists into a
  new splitListOfPairsIntoPairOfLists() method for reuse in a follow up
  change.

Bug: 171479578
Test: m nothing
Change-Id: I824ed1bc673b47757424563fc37e1190adff2ed1