Declare critical exposed methods of ObjectToPack final

Declare critical exposed methods of ObjectToPack final

There is no reasonable way for a subclass to correctly override and
implement these methods. They depend on internal state that cannot
otherwise be managed.

Most of these methods are also in critical paths of PackWriter.
Declare them final so subclasses do not try to replace them,
and so the JIT knows the smaller ones can be safely inlined.

Change-Id: I9026938e5833ac0b94246d21c69a143a9224626c