Move PrimitiveData serialization back from AbstractPrimitive

[taylor.smock]

In 19078/josm:

Fix #4142: Track fully downloaded objects (patch by stoecker, GerdP, and myself)

The serialization move from PrimitiveData to AbstractPrimitive should be
reverted prior to 24.05 (see #23677).

The serialization move was required since we want to ensure that all downstream
users of AbstractPrimitive were not using the flags field, which was done by
making the field private instead of protected. They may still be using that
field (via updateFlags) which would not be caught by compile-time or runtime
errors.

Additionally, a good chunk of common functionality was moved up from
OsmPrimitive, even though much of it wasn't useful for PrimitiveData.

[stoecker]

Either we do this before or after the release ;-)

[taylor.smock]

In 19096/josm:

Fix #23677: Move PrimitiveData serialization back from AbstractPrimitive