Deal with ojluni files that include changes beyond OpenJDK 8u121-b13.

Base64: document a change that was integrated from beyond 8u121-b13.

JceSecurity: un-integrate a redundant change so that this file is now
  based on upstream OpenJDK 8u121-b13, like most other ojluni files.

DefaultProxySelector: document generally that this file includes
  changes beyond 8u121-b13.

JarURLConnection: document a change that was integrated from beyond
  8u121-b13.

FtpClient: document a change that was integrated from beyond 8u121-b13.

Bug: 62368386
Test: Treehugger
Change-Id: I1605a979d9ba650346a4fcddeb102cc5644de2c1