ICU-23354 Remove m_utilIntBuffer_ and m_utilStringBuffer_ from UCharacterName.AlgorithmName

The `m_utilIntBuffer_` array in `AlgorithmName` is a 1KB `int[256]`
that was eagerly initialized, leading to unnecessary memory
consumption for instances that never use it (e.g., `TYPE_0_` instances
or unqueried algorithms).

Use a local variable instead for m_utilIntBuffer_ and
m_utilStringBuffer_.

Author: gvictor