Standardize use of direct variable access vs accessor methods in IonValue and subclasses

[tgregg]

          In some places we use `_child_count` directly, and in others where we could we instead use `get_child_count()`. What governs the choice?

Originally posted by @jobarr-amzn in https://github.com/amazon-ion/ion-java/pull/557#discussion_r1320162678

[tgregg]

Also examine use of accessors for IonValueLite's _flags, e.g. isNullValue(). See: https://github.com/amazon-ion/ion-java/pull/546#discussion_r1286329817