[avalanche] Move the processor updatedBlockTip logic to a private method

Summary:
This makes it possible to test the handler from unit tests, without having to setup a full validation interface.

There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14581