Better verify java.util.stream against OpenJDK 8u121-b13.

This change:
 - Restores some unused imports that are present upstream, to minimize the diff.
 - Removes an unused import that isn't present upstream but was used in a
   previous (now-reverted) Android change.
 - Rewords a comment to avoid some long lines.

Bug: 35910877
Test: make
Change-Id: I5a1a6ef77a17d8d245aef2986114302441de4908