ICU-21763 UVector cleanup in vtzone.cpp

ICU-21763 UVector cleanup in vtzone.cpp

Revise uses of UVector in vtzone.cpp to better handle memory allocation
failures.  This is one of an ongoing series of commits to address similar
problems with UVector usage throughout ICU.

The changes primarily involve switching uses of UVector::addElementX() to the
new adoptElement() or addElement() functions, as appropriate, and using
LocalPointers for tracking memory ownership.