ICU-21662 UVector cleanup in rbtz.cpp

ICU-21662 UVector cleanup in rbtz.cpp

Revise uses of UVector in rbtz.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 include

- Use LocalPointers and UVector deleter functions to simplify OOM checking and recovery.
- Fix RuleBasedTimeZone::addTransitionRule(rule) to have standard ICU adopt behavior
  when errors occur, meaning automatic deletion of the incoming rule. This simplifies
  both the implementation of the function and the code at the call sites.
- Update addTransitionRule() call sites. Includes modifying the Dangi calendar initializtion
  to not silently ignore errors.
- struct Transition is changed to derive from UMemory, which allows the use of LocalPointers.