[wpilibj] Add Preferences.getNetworkTable()

[kcooney]

Use case: Our team has a shared library that includes wrapper APIs for the
Java Preferences API. The wrapper API adds a topic (with a topic name starting
with ".") where it stores non-persistent data that is used for validation.

The code for our wrapper API is here.

The unit tests for those classes use Preferences.setNetworkTableInstance()
to ensure that the tests are isolated. Since there is no current API to get
the current preference table, our wrapper APIs either have to take in a
NetworkTableInstance (which makes the classes less simple to use) or our code
to add these internal topics needs to use the default network table instance (making the
tests not completely isolated).

Providing an API to get the NetworkTable used by Preferences would allow our
API to remain simple and our tests to be truly isolated. It doesn't appear
to be something that would be hard to maintain.

I reviewed the C++ and Python APIs, and they appear to not have an equivalent
to setNetworkTableInstance() so this PR only updates the Java API.

[kcooney]

@PeterJohnson would you be able to review this?

Thanks!

[kcooney]

@PeterJohnson friendly ping

[PeterJohnson]

Can you add this to C++ as well?

[kcooney]

Can you add this to C++ as well?

@PeterJohnson I am not familiar with the C++ code. Is there a C++ equivalent of Preferences.setNetworkTableInstance()? If so, could you point to where it is defined?

I couldn't find one, so I didn't see the same need for a C++ equivalent of the API I am proposing (in the C++ code, preferences appear to be always stored and read relative to the root of the network table default instance).

Be happy to add Preferences::GetNetworkTable() if I am misunderstanding something.