Run system_server benchmarks for 5 seconds

Especially for short running methods, this gives us better results and also
gives us more data for things like simpleperf to work on.

Since we don't know ahead of time how many times each benchmark will run, we had
to switch the mean and standard deviation formulas to one that supports running
updates. This has the advantage of making it constant space.

Bug: 140743821
Change-Id: I3e6b6b44d3bd573bad8afc8831226194147b108a