Refactor ProfilerSimple

Refactor ProfilerSimple

This patch refactors ProfilerSimple, moving the code that creates a new
profiling entry and the code that updates an existing entry to discrete
methods. This allows subclasses to supply a different implementation. It will
allow me to introduce a profiler class that uses RunningStat (introduced in
Ifedda276d) without breaking existing APIs and without having to duplicate lots
of code.

Change-Id: Ida7d7d0c1e2a98618b51246861e6af8ec3eb6320