Replace usage of Java's, JGit's and Apache Commons' Base64 classes with
BaseEncoding from Guava.
No Gerrit code directly depends on Apache Commons Codec any more, so we
can remove all the explicit dependencies from the build rules. However,
it is still a transient dependency of the httpclient library, and is
used by some plugins, so we can't completely remove it from the build.
Change-Id: I9a934bda3e4519f359759b20efb298d544f1391b
a6270b9506