Move rho_ relevant codes from Precompute to SolveOneIteration.
Since rho_ is adaptive in each iteration, we move codes relevant to
rho_ from Precompute to SolveOneIteration in AdaptAdmmFixedSolver.
We test it by comparing the results with AdmmFixedSolver. Since we
don't implement adaptive rho yet, the results should be the same,
which turns out to be our case.