Verify misc java.util classes against upstream OpenJDK8u121-b13.

Verify some misc classes that only require minor adjustments.

*SummaryStatistics: Reintroduce unused import. After this CL,
   these file are exactly identical to upstream.

OptionalInt: Revert addition of an empty line.

Scanner: Refactor to merge to separate changed blocks into one,
   tweak change markers.

Vector: Rephrase change marker and document additional places
   touched by this change.

Bug: 35910877
Test: CtsLibcoreTestCases

Change-Id: Ie71a8c6b0acc5d3c8926bd405ae4fc37ba517d4e