altoslib: Store common frequencies in library version-independent form

altoslib: Store common frequencies in library version-independent form

Serializable Objects in java are very specific to the class being
serialized. As we bump the name of the library on a regular basis to
note API/ABI issues, this mean a saved a Serializable object in
the preferences database will fail to load across library version
upgrades.

The saved tracker state and saved common frequencies were the only
objects saved in this form; this patch adds infrastructure for writing
objects in a version-independent form, and then adds support for
saving frequencies in that form.

Signed-off-by: Keith Packard <keithp@keithp.com>