Get time less often in AddrManImpl::ResolveCollisions_() #25224

This makes the code less verbose. Also, future changes that change how
to get the time are less verbose.

Moreover, GetAdjustedTime() might arbitrarily change the value during
the execution of this function. For example, the system time advances
over a second boundary, or the network adjusts the time arbitrarily.
Most of the time however the value will not change, so it seems better
to always lock the value in this scope for clarity.