gui: Set CConnman byte counters earlier to avoid uninitialized reads

Summary:
Initialize CConnman byte counters during construction, so GetTotalBytesRecv()
and GetTotalBytesSent() methods don't return garbage before Start() is called.

Change shouldn't have any effect outside of the GUI. It just fixes a race
condition during a qt test that was observed on travis

This is a backport of Core PR17906

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8429