Make the Evaluator statistics key strings consistent

We keep track of evaluator call and time statistics
via a hashmap containing magic strings. These strings
need to be consistent across the GradientProblemSolver
and Solver as the LineSearchMinimizer is used by both
of these solvers. Previously they were inconsistent
in a manner that GradientProblemSolver was not getting
information about the evaluation timing, and in the
process of fixing that I made it so that the TrustRegionMinimizer
when solving bounds constrained probelms will access/update
this information correctly.

So while I look for a more elegant solution, this CL
is meant to fix the inconsistency by making sure that the
same magic strings are used everywhere.

Change-Id: I120ca0bd1c2f77fde2db15edd9e33286a49dbae9