ICU-10347 add U_FINAL and U_OVERRIDE for C++11 "final" and "override".

ICU-10347 add U_FINAL and U_OVERRIDE for C++11 "final" and "override".

Added them to several "not for subclassing" classes.
Some exceptions:
* Commented out U_FINAL where there's an internal override needed.
* For tests, added "#define U_FINAL" at the top to disable this.
#defined to null for Doxygen.

X-SVN-Rev: 36177