Changed speed optimization for strings.

Removed use of StringUtf8Micro and instead use an extra byte array
instance variable directly in the class. This allows the list returned
for repeated strings to be a String instead of a StringUtf8Micro
making the class compatible with existing code.

Removed PerfTimer.java which isn't used.

Change-Id: Ie6acfb40f98f59a48c1a795d86f715078f9611f5