StopWatch: Declare a stop_watch variable while timing

* A variable is needed to hold the reference of stop watch
  in the current scope. The variable will be freed once the
  current scope exits
* Without such a variable, the constructed stopwatch will only
  exist as a temporary stack variable and freed immediately
  after the declaration line, making the stop watch only timing
  the time needed for construction and immediate destruction.
  As result, current stop watch all show very minimum durations.
* Commands used:
  * find ./ -type f -exec sed -i 's/StopWatchLegacy(__func__);/StopWatchLegacy stop_watch(__func__);/g' {} \;
  * find ./ -type f -exec sed -i 's/common::StopWatch(/common::StopWatch stop_watch(/g' {} \;

Bug: 207608879
Test: GD presubmit and make
Tag: #gd-refactor
Change-Id: Ia38cf3d1473f750c6a08ba4d6551d7fafba4345a