Drop unused libcore.io imports.

This removes the last import of libcore.io.Libcore from frameworks.
This means that it can be safely hidden from frameworks in a follow-up
CL. Both of the unused imports in this CL were already removed on the
internal master branch (commit a05a9946),
but that commit doesn't cherry-pick cleanly onto AOSP master.

The IoUtils import also appears unused, but it wasn't touched by the
above CL so this CL also leaves it alone.

Bug: 117548625
Test: Treehugger
Change-Id: Icc68c64ef2907372af848d74d9cf96070c89d52b